Can I change the venue of a post-decree case, which started in County A. . . to County B?

This is like a law school hypothetical. Family law is governed by state law so you need Ilinois counsel. But here are some general comments:

The county where the divorce was finalized and the last JPA filed often retains continuing jurisdiction to resolve subsequent disputes, particularly involving children and child support. The divorce decree sometimes contains a clause that says exactly that.

Whenever a party to the agreement moves to different county, disputes can arise over which court should now exercise jurisdiction. What is unusual about your case is that you want to proceed in ex's new county of residence rather than your own.

Most people want to litigate in their own home county if for no other reason than to save on travel and expenses.

The burden will likely be on you to persuade both courts why B should now take case, especially if A has already made any substantive rulings. There may be some odor of forum shopping that neither court will like. Have you tried the old BRIAR RABBIT? Tell anyone who will listen you would greatly prefer to litigate in A because if case got moved to B it would be may more inconvenient and expensive for you. Whatever happens, you sure dont want to end up in B's bramble bush.

It sounds like you got some intell on opposing counsel. Intel is always good unless its inaccurate which it often is. What is the source? Perhaps more importantly, is an attorney who will only appear in one county someone to be feared? You could find that out real easy by conferring with local A counsel which you really should do anyway. You have to be careful that this "fantastic lawyer" who will only practice in A is not running his own BRIAR RABBIT.