yes when u update the sign or admin allows to add sign to new user after some post. the sign which one is created will shows on all the new and old post. thanks etatvasoft web developer at web development company www.etatvasoft.com
as per my knowledge, the signature link is for the account not for the post. if you change settings to your account, automatically your details will be updated in the posts.
I think its good. This way it can hinder spammers or stop the ability to gain lots of backlinks through changing the sig to contain links-thus all previous posts contain the backlink.
Yes once if you have changed your signature, that will be replaced to the older posts already you have done and changes will occur in your further posts. But the back links and rankings for the previous signature will drop.
You have got your answer from other members. Just wanted to drop a note that you must not often change th signatures. This will create a uncertain links in your signature and the links will loose their credibility over a period of time. Do give some time before you change!