[Lynne]

Can't you do this:

The user registers and gets put into the Registered Users group. This is the group with permissions you want to give the Trial user and has almost all (perhaps even all?) the same permissions as the Subscribed User group.

Make another usergroup that has limited access (Limited Access Group?). This is the group you want them to be moved to after the free trial and that does not have all the neat privileges as the Subscribed User group.

Make a Promotion (or in this case it will be a Demotion) that takes all users from the Registered User group and dumps them in the Limited Access Group after they have been registered for a month.