Membership cap and register button

Ok,
So I dont know how to go about this. All users who want to access our site must register, so they are already registered to the site and can view the site, now i want a button for some content like(register for content) where its just a button that upgrades there membership , instead of having to do a whole membership form again. is this possible to just have a button that upgrades or changes there membership to a different membership type, assuming i want all of the membership to be free. how could i do this?? also is it poissible to say close registration for certain content once the number of members for that content say reaches 100 users??? not allowing anyone to register for that content(lets call it a registration for a contest to give a real world example) closing the registration ergo not letting them become that membership anymore and deactivating that registration button??? so its 2 part question, 1. providing just and upgrade or change membership type button, 2 capping the number of members allowed to upgrade or change their membership type once a certain amount of people belong or have signed up to be that member level.