hrm, thats a toughie..I dont think you'll be able to downgrade and keep your list, unless you can find a program that will help you. there are programs that will backup your list so you can transfer to another pc, or whatever, but I Dont know if it will convert it to 99b format.
and yes, there are several icq clones around for windows, even more for *nix, but I just stick to icq 99b, works like a goddamned champion.
also - what I do periodically is copy my entire icq dir. to another folder and periodically to a cd (keeping my list and ALL my history of events is critical for me). and it works fine like that, when I need to format I just install icq 99b regular, run it, let it register my #, then exit icq and copy over the icq folder with my backup copy, and let it replace everything, and bam, back to where I was.
good luck