Request for Plan Upgrade and Prorated Credit

Hello ,​I currently have an active €3.99/year (Special Tier) plan, but I am experiencing frequent "Panel link lost" and high latency issues because I am running a Lavalink music bot which requires more resources.​I would like to upgrade my server to either Tier 0.01 (€9.99/year) or Tier 1 (€11.49/year). Since I already paid for the yearly plan, is it possible to upgrade by paying only the price difference (prorated)?​Please let me know how I can proceed. Thank you! I opened the ticket a long time ago and haven't received a response.