How do you get the B side back "clean"?

Let's say I want to apply the latest OS/400 cume, but I can't run the risk of 
causing application problems.

So before installing the cume, I apply all my PTFs *PERM. Then I apply the new 
cume.

Now in theory, if I have a problem under the newly applied maintenance I can 
IPL from the A side and everything should be back the old way, correct?

But how do I get back to running on the B side? I guess what I'm asking is, 
isn't my "B side" corrupt now (it contains code that causes problems) and if 
so, how do I get my system back to running on the "B side", or do I even need 
to, etc?

Is there a process for copying the (good) A side back over the (bad) B side to 
make B side good again?

Hopefully someone can tell what I'm trying to ask.

Thanks,
-Marty