Yes you can purchase the subscription but all the billing will work as per the date you purchased the subscription and not when you install/activate it.
can I install on more than one computer? Or switch from a pc to a Mac?
You may install software on up to two computers. These two computers can be Windows, Mac OS, or one each. If you install on a third computer, it will request you to de-activate on the other two computers. You can then reactivate one of the previous two computers, and use Creative Cloud apps on it.