Can someone explain to me David Hume's principle of conversion with the following examples? When viewing a well-written tragedy as a work of art, two movements are created in the soul, or mind, according to Hume. On the one hand there is what he calls the predominant movement, which is caused by the genius, eloquence, beauty, and talents displayed in the tragedy along with the fact that the tragedy is an imitation, i.e. is unreal. This movement might reasonably be said to be the appreciation of the aesthetic merits of a work of art. Along with this movement in the soul there is, when viewing a tragedy as a work of art, a subordinate movement which is the result of affected passions. This latter movement is a negative, or disagreeable, one owing to the kind of passions that arouse it in the spectator, e.g. sorrow, indignation, woe, etc. The Principle of Conversion is the principle whereby the subordinate movement is converted into the predominant movement, and upon such conversion the latter is strengthened. The result of this conversion is an increased feeling of pleasure, or delight, in the spectator of the tragedy. To help validate his theory concerning the problem in question, Hume offers six examples of instances where the Principle of Conversion is operative. In his first example, novelty, or the feeling of 'newness', is considered a subordinate movement which gives strength to whatever emotion is predominant, e.g. joy, sorrow, pride, shame. In the second, the predominant movement of jealousy in Othello is increased by the subordinate one of aroused impatience. Examples three, four, five and six are cases where the predominant movement is increased by a subordinate movement which is occasioned by situations of difficulty or adversity. In three, the feeling of affection in parents is said to be strengthened by the difficulties involved in rearing a sickly and weak child, so that the parents commonly have greatest affection for him or her. The fourth is a case where the sorrow caused by the death of a friend increases the sentiment of endearment which was felt prior to the friend's death. In five, jealousy and the uneasiness caused by absence of a lover are viewed as the cause of a movement which helps the agreeable affection of love to subsist. Finally, example six is a case where esteem, or being prized, is given such additional force by the grief for the artistic creator who struggles with death that one prizes his last unfinished work most. Although one might (perhaps justifiably) take issue with certain of the technical points of Hume's examples- for instance, by suggesting that a parent's sympathy for a weak and sickly child accounts for his or her additional affection, or that one often has merely additional appreciation for the final works of artists struggling with death-I think that the main point he is attempting to establish by them, viz. that what he takes to be a predominant movement is strengthened by the conversion of a subordinate one, is quite discernible.

When asking for clarification as to what Hume's principle of conversion actually means and entails, it's useful to get a larger sense of Hume's discussion in "Of Tragedy." What question is he asking? Who is he in conversation with? What is he trying to explain to begin with? What's the focus of the essay?
With that in mind, Hume begins his essay "Of Tragedy" by noting a phenomenon quite common to dramatic tragedy: often, viewers tend to receive greater degrees of pleasure and enthusiasm from more intense displays of sorrow and suffering. Of course, this raises his question, how is it that these extreme displays of suffering can evoke this response? He cites one Abee Dubos, for one particular explanation, which suggests that the real enemy of human happiness isn't so much unhappiness, but rather apathy, and any kind of emotional response (whether pleasant or unpleasant) is still to be held as better than the lack of one. However, Hume is unconvinced by this explanation, because it does not explain how this negative emotional response is transformed into something positive. Even if it is better to feel sorrow than to feel nothing at all, this still doesn't explain how people can turn sorrow into happiness. Next, he cites one Fontenelle, who argues that emotional responses are ultimately interchangeable, so that one can be transformed into its opposite. As an example, one can turn pleasure into pain and pain into pleasure. One can find joy in sorrow, and one can find sorrow in joy. This logic forms the basis for his theory of conversion, which you ask about above.
From here, he comes to the role of oratory and of artistic expression, which serves as the medium through which skillful writers/poets/artists can manipulate the emotions to achieve a desired effect. As he here writes, "this extraordinary effect proceeds from that very eloquence, with which the melancholy scene is represented." Thus, a great dramatist or artist can use their artistic abilities to create this effect, by which sorrow is transformed in the mind of the viewer.
From here, we come to his examples, which serve as illustrations of his theory. What's interesting is that these examples are meant to illustrate ways in which one emotional reaction can be converted to another. In any case, these examples are ultimately drawn from real lived experience, as examples by which one emotional reaction (usually negative) can be converted into another, more positive emotional experience.
The first example is Novelty, which I would suggest reads a lot like suspense. As Hume himself writes,

Had you any intention to move a person extremely by the narration of any event, the best method of increasing its effect would be to artfully delay informing him of it, and first excite his curiosity and impatience before you let him into the secret.

From here, he draws on the example of Othello and how Iago uses this kind of technique to intensify Othello's jealousy, by playing on impatience. By holding back information, by keeping a listener in suspense, a person can more effectively maintain their interest and create a much stronger emotional reaction in them as a result.
As further examples of this Principle of Conversion, Hume mentions the role by which "difficulties increase passions of every kind; and . . . they produce an emotion, which nourishes the prevailing affection." He gives an example of parents with a sickly child, who could be expected to cherish that child all the more greatly as a result of their uneasiness concerning the child's health. Fear and anxiety are here converted into love.
He mentions the role by which a recent death can result in far warmer sentiments toward the deceased on the part of the survivors. He uses the example of jealousy and separation, which can actually intensify affection and strengthen relationships. Ultimately, through the examples, Hume suggests that his Principle of Conversion is actually based in reality and can be seen as a facet of nature and of human behavior, and if we can see it expressed in the real world, we should not be surprised to find the same principle at play on the stage.
(Do note, Hume's essay does not end here but continues onward, but this does cover much of reasoning which your question asks to clarify. I hope it has proven helpful in this task.)