Ah, Access has changed buffer sizes (again).
In any case, KKilFoil's post correctly points out that optimistic and pessimistic locking are contributors. You need to check the database settings for the type of locking involved, and also need to check the lock settings for the forms involved to see if they have too wide a scope on the locks they set. You CAN (but should not) set a form to NOLOCKS. There are RECORD and TABLE lock to consider as well.
KKilFoil reminded me of the other factors. A change to key style (from autonum to something else) shouldn't interact strongly with these settings unless TABLE is selected for lock scope.
The Help files have some pretty good articles on setting locks in forms and queries, and either could be the source of the problem. You could ALSO be accidentally setting a table lock on a read-only lookup table, which you should check as a possible explanation, but again, a change in key style should not interact with such a setting.
Unfortunately, there is a wild-card in the deck. WINDOWS communicates the updates to the LDB file which is where lock info is kept. If you do something that is relatively slow or requires access to a single item (the TableDef, my original point of focus), you are updating a lock structure in the LDB. The slower this operation (the more disk I/O is involved), the longer the lock is engaged. And the longer the lock is engaged, the higher the odds that some other person on the same mission will encounter the lock.
Changing the key structure to something that somehow is either faster or doesn't have to open the TableDef in update mode SHORTENS the time that the lock is engaged in the LDB file, which lowers the odds of encountering a locking conflict.
Therefore, my idea of what is going on remains the same, but KKilFoils article prompted me to point out other factors (lock size, lock type) that affect the odds of locking encounters. In the final analysis, it is your problem. All we can do is offer guidelines and idea. I hope I have given you a good one.
Thanks for your kind words. My normal place of work is a Dept. of Defense office that blocks IM at the perimeter firewall. I can turn on IM or not, it won't matter. (Actually, it WILL matter. I'll get lectured.)
I don't turn on IM on my home machine because I work as a security manager for a large applications/database machine and am too intimately aware of what happens when IM is turned on. So even though you have absolutely good intentions, my home firewall is still going to block IM too.