I live in New Zealand, and our second seat of Solid Edge is done via a monthly subrcription.....the service is very easy [obviously, thoroughly read through all of the information that is sent] to set up, and renewal emails are sent every month, so you know ahead of time, before it runs out.
I have ours set to "auto renew" via credit card payment....so the system emails me that it's about to happen, then that it has happened.
Design Manager Streetscape Ltd
Solid Edge ST10 [MP3] Classic [x2 seats]
Windows 10 - Quadro P2000
I would assume you just upgrade to the Premium license if you need the add-ons, then downgrade once you are finished with them.
I was thinking that myself. But comparing buying a license and paying for maintainence - this is gold. I don't think SolidWorks offers this. You can't even get a trial version. It says it on their site - but its BS - cause I tried. I believe Pro/E offers it? Have to check.