How do I transfer existing 7.0 to 7.1 site without double paying for subscription?

I'm moving my existing 7.0 site to the 7.1 platform. The instructions say that I have to pick a 7.1 template and then upgrade it to a paid service. I've picked a 7.1 template, but If I upgrade the 7.1 trial to a paid service, then I'll be double billed as I've already paid for the year for my 7.0 site. 

I'm surprised there is no instruction about this part. How do I avoid being double billed?

Please help! Thanks :)

Depends on how long you have had your 7.0 site. I suggest you read this before moving:

https://support.squarespace.com/hc/en-us/articles/360038270572-Moving-from-Squarespace-version-7-0-to-version-7-1