You can move between Creator and Professional at any time from your account dashboard.
10.3.1 Upgrading to Professional
Immediately unlocks Professional-only features — additional artist profiles (up to 10), live chat support, Release Protection, and more. YouTube Content ID, Cover Art Studio, and royalty splits are already included on Creator. See 1.3 for the full feature list.
10.3.2 Downgrading to Creator
Takes effect at the end of your current billing period. Professional-only features become unavailable once the downgrade applies. Existing releases keep whatever Release Protection status they've already earned (covered under the plan-transition rules referenced in 1.3 and 1.4) — downgrading does not retroactively strip protection from already-protected releases.
10.3.3 Proration
Upgrading mid-cycle is prorated for the remainder of the current billing period; the full new-plan rate applies starting the next renewal.
Source: MadDesk article 10.3 (10-3)