...if I buy a cab that has a 15 in it, I would really like it to be 31khz...
uhm... you can put a 31 in any CABINET, but the game you hook up to it MUST be 31 as well. Just like a cab with a 15 monitor can only run a PCB that runs at 15.
So, yes it is very easy to replace a monitor in a cab, you just have to be careful which boards you connect to it.
Not trying to be a smart a$$ by repeating what I said, but it seemed to me you were asking the same question again that I answered in my last post. Just trying to make sure you don't ruin a new monitor

Not that you sounded like you were going to buy one until you had this question answered!
If I didn't answer your question right, please try asking in a different way and I will try to answer it in a different way!
DUH! re-read your post... got it now... forget everything but my second paragraph and add that you might need to make sure that the monitor is the same size to make replacement eaisier... (19" for 19" tec.)