Just let us know (team AT appveyor.com) your account name and
the plan you'd like to upgrade and we'll upgrade subscription
proratively for you.
If you are close to the end of billing cycle and want to change
billing details or upgrade just cancel subscription (it doesn't
remove/change your projects - it just switches your account to a
free plan) and purchase a new plan - internally your customer ID
will remain the same.
When a payment for the next period is failed (CC changed or
expired) your account will be switched to trial on the same plan
and will remain in this state for 5 days allowing you to
re-purchase a plan with different credit card.