If I upgrade my old Classic forum to Threads, what happens to member #1 (me)?

You become member #2. Member #1 is reserved for technical stuff like anonymous posts.

So what happens to number 2? Do all members shift up by one?

Probably not a big deal for most places, but unfortunately I built an item years ago that is the most popular thing on this particular website, and the numbers are tied to the member number. Thousands upon thousands of links out there on websites going to particular members. Site is http://map.datastormusers.com and one of 4 permanent links to my info on the site as an example is http://map.datastormusers.com/user3.cfm?user=1

It is even worse in that most of the third-party tools that allow posting to this map use the forum cookie to identify the member

You know, that is a good question.
I never paid attention when I upgraded from classic a few years ago. I was to involved with that the upgrade worked to notice what happened to the other member numbers.
Looking at my member database it looks like your assumption is correct.

Looks like you are using mappoint for the one item above.

Just at a quick look how it works. It looks like the map would update as users report in so that would be a headache for a short period of time as people report in. Because they would have to login again to establish a new cookie. But I would be more concerned on the cookie issue . How it is handled now and how it would be handled with threads!

You are correct that I'm using mappoint. The cookie differences would be a headache in rewriting utilities, but a shift of number would be impractical in the extreme. If someone has a web page with hundreds of links to their location, as many of my members do, all of those links would now point to someone else. Most use it in e-mail signature lines, and of course in forum signatures.

The requirement to join the forums in order to show up on the maps has been very helpful in making sure that the maps don't get spammed. I would hate to break the link between membership numbers and map id numbers.

About 2500 of my members have placed themselves on the map at one time or another - many drop off when they aren't actively travelling.

In the stock code there is no getting around the change in member number. Maybe Rick or one of the beta testers can offer some special installation.Might cost a few but it looks like that is what you need. They may for instance just bump you to a new member number and let all the others stay as is. I just am assuming.
Because member 1 as stated is reserved for pretty much orphaned posts.

Except for user #1, which is pretty much reserved, it would not be an extreme difficult job to renumber everyone back to their old member number in threads.

Most time would go in being careful, not the actual doing of it. You would need a little modification in the migration tool to not throw away the temporary tables it uses for the migration. There is table called ubbt_memtemp used to remap users from old to new. You could use that to get the usernumbers back again.

As far as the numbering goes, by default it starts with #2, and just numbers up. So depending on the number of deleted/missing users, the new member# can differ a lot from the original.

Yeah, not a real slick solution to this one and it would definitely take a custom import. Someone would have to be put to the back of the pile. Meaning, either your user #1, or some other early user would need to be given a user number at the very end and then the importer modified for this so it doesn't increment everyone else.

The classic_import.php script would be what needs modified. If it's not something you're paricularly keen of tackling yourself I'm sure there are a few people here that could assist including myself in my off-time.

Yep it looks like he has a lot and I mean a lot of archived static pages built based on member number, It would be a monumental task to try to rebuild them because a member number changed by one or two.