When you upgrade your Foxy plan, your existing credits stay in your account and the new plan's credits are added on top.
The full picture
Your existing unused credits stay in your account
You immediately receive the full credit allotment of your new plan
Total balance = your old credits + new plan's credits
About the upgrade charge
Upgrades are not prorated. You'll be charged the full price of the new plan when you upgrade — not just the difference between the old and new plan. In exchange, you immediately get the new plan's full credit allotment, and your billing cycle resets to the upgrade date.
Want to upgrade?
Open Profile menu → Change plan, pick your new plan, and confirm.
For more on how upgrades and downgrades work overall, see How do upgrades and downgrades work?