You can change out banners and text links on them anytime as long as you don't send any links to pop up h*ll or add blind links etc. We did it all the time and even emailed some tgp's to tell them and their response was: follow our rules and we don't care about what you change as far as the advertising. They realize that the banner you THOUGHT was great might s*ck so change it!
We get them spidered and all we change is the ads etc. Leave the pics up forever and just keep going with others.
BTW, the old man has never been blacklisted, from 1995 on going this way. We do NOT however change the actual site advertiser, just the banners or texts.