Stripe, Paypal or cc
Yes you can, any applicable payments will be completed to the nearest month. At which point all users will lose access to the portal and the data.
If you start on the Basic package, you can move to the Essential package when you are ready.
The costing model is pro-rated so if you are ready in 6 months, you would just pay the difference for the last 6 months of your current subscription.
Upgrading can be done yourself within the portal.
Note, you do also have the ability to downgrade at any time, features within that package will just sit in the background and you won't lose any content in case you decide to upgrade again.