Switching blog template

Hello,

Is it possible to change the blog template after creation.

I have a free blog template that I offer users supported by adverts. I would like to change the template easily when they pay.

The setup is latest wordpress and plugins.

I have a mainsite and then subsite for e.g. gifts.com. My customers on gifts.com can get a free site to show off their gifts which I setup using the template tool - so they get shop1.gifts.com. This is supported by adverts which is part of the template we have setup.

When they want the pro service, I would like to use another template with no adverts and map their domain.

How do I change from one template to another? I do not mind going into the database to change the entry if there is no easy way to do this. I just need help finding the right things to do.

PS - Is it easier to create a network of network e.g. mainsite network then subsite network.

Regards