Learn English – an antonym of improved/improvement

"Happy [is] the man who listens to wisdom." means "The man who listens to wisdom is happy." This is a common proverbial form and is usually used to give advice:

Wealthy is the man [...]

Healthy is the man [...]

Happy is the man [...]

This may be easier to understand if you use "will be" instead of "is":

Happy will be the man who listens to wisdom.

"[...] who changes Emma for Harriet" means "[...] who exchanges Emma for Harriet." To rewrite the entire sentence:

I mention no names; but the man who exchanges Emma for Harriet will be happy!

Since we don't have the entire context, I don't know if the exchange has already happened or if the speaker is suggesting that the man should exchange Emma for Harriet but the gist of the sentence is that whoever is courting Emma would be happier if they were courting Harriet.

I want to make a distinction not well covered by the other answers. In your example definitions, we find two different (very different) sorts of verbs: v.tr and v.intr vs. aux.v. It's the same word shape, but the meanings are almost entirely separate (you can sort of see how the one led to the other, but this is not relevant aside from historical interest).

aux.v is the auxiliary, or modal will, when it is used to as part of a larger verb form, such as will be, or will have been built. This is not the word in question. Exclude all of these from your choices.

We are thus left with:

v.tr.
1. To decide on; choose.
2. To yearn for; desire: "She makes you will your own destruction" (George Bernard Shaw).
3. To decree, dictate, or order.
4. To resolve with a forceful will; determine.
5. To induce or try to induce by sheer force of will: We willed the sun to come out.
6. To grant in a legal will; bequeath.

v.intr.
1. To exercise the will.
2. To make a choice; choose.

or more simply:

tr. & intr.v.
To wish; desire: Do what you will. Sit here if you will.

Any of these may be applicable, depending on context, though some are more common than others (generally, the higher ranked are more common).