First, find your negative Tab balance. This needs to be repaid in order to upgrade. If you go a store they will apply it to your next bill.
The good news is you can keep your plan and take even Tab M (just add $5 to your plan for 24 months), which takes more off the phone you want. The 5C is $625 - 300 = $325. It's $475 with Tab S but you don't need to pay the extra $5.