What Dani says is exactly true. One's should not messing around with signature link. I recommend to decide your keyword(s) first by analysis then put your signature. Then you don't have to change your signature more often! It's easy, Isn't it :)
You know, once upon a time forum signatures and forum in general were almost always very useful. There's maybe 100 Purposeful Forums on the entire web and the gazillion rest just dumpgrounds pretending to be like purposeful forums.
To think Google isn't aware of a signature link because it is "no-followed" is like the time I walked to bus stop to get to work but forgot to put my pants on. It didn't matter to me until I noticed. I'm sure others noticed first. I guess I'm off track.
Point is, if a user participates in a meaningful discussion within a forum then it's usefulness to either learning or teaching has more merit than whatever miniscule ranking reward the links in the participator's signature could conjure, if any at all.
Whenever I run into an intelligent appearing spam forum that Google has yet to devalue, it gets blown out by some clever link starved marketer so fast it's hardly recognizable as anything remotely meaningful within a week.
No i dont think so because whatever posts you have done earlier will be crawled & indexed by search engine so if you change the forum signature and post new comment nothing will happen to your old posts backlinks.