More questions:
1) Do I have to buy a MYW hopper?
No, and you can buy a pass with water park options, but if you use any of those water park options, you can ONLY upgrade to a PAP, not an AP.
2) How many days of MYW do I need to buy to achieve the greatest discount?
The amount of discount available changes from time to time. At least, I think it does - it seems like the pass I bought last time doesn't offer as big a discount this time, anyway.
As I said above, pick the pass that has the best discount off gate price that also isn't more valuable at gate price than the AP.
3) Can the MYW pass be the expiring kind?
Yes, it can be any kind of pass.
If there is a link to an existing thread with these detailed instructions, that would be great, and I apologize for rehashing all of this.

This is the first time in ten yeats that I don't have in hand an AP to renew, so I'm starting fresh and wwwaayyy confused.
I don't know that there are any instructions, but look for the Everything about Tickets thread on the Theme Parks board, posted by Cheshire Figment. It has a great deal of information about upgrading tickets.
It really is easy to do, BUT you have to be sure of your calculations - how much your pass will be worth when you upgrade it, and how much you will owe on the new AP. Not all CMs do this transaction smoothly and some will end up trying to charge you more. In my case, I was supposed to owe $51 total on an upgrade to 3 APs and the CM at first tried to tell me I owed over $200!

I explained to him that was wrong and he did the transaction again (with help) and got it right.