For some reason, I could have swore that Slackware 9 came with both versions, but I just checked my laptop and was wrong. I've been using 9 for a week or two now, and have compiled most of it by hand, and so far I haven't had any problems. I am willing to bet that unless you are trying to compile a really old program, most everything should work just fine, and since you updated to Slackware 9, everything that that you installed was compiled with 3.22, so anything new that you compile will interact with out any problems.
So, all in all you should be ok.