This doc explains how to upgrade a VPS to a higher plan, which can include more RAM, additional vCPU cores, more disk space, and increased monthly traffic commitment.
You only pay the price difference between the two plans, prorated for the rest of the billing period.
You don’t pay the full price of the new plan — just the upgrade difference (you don’t pay twice)
If you are already on the largest tier, and you’re running out of available traffic, you can purchase traffic. EDIS Global offers the possibility to maintain a traffic pool. A traffic pool allows you to manually assign traffic to your VPS, or enable auto-hydration and automatic refill at 99% usage levels.