Everytime an affiliate signs up for a program it's basically nothing but a handshake deal. Read the "agreements" and you'll see 99% of the programs have in there they can change the terms at any time
I haven't made a deal in writing since I've been in this biz and haven't been screwed over yet. Just did another one last week via nothing but ICQ (VERY part time so don't anyone think I'm really getting back into the biz ;) ). Once you've been around for a while, you know who to trust and who not to trust.