I’ve been looking into myself and I’m hopeful that it won’t be too difficult. The reason I say this is because Professional is actually built on the same codebase as Community, and as far as I know the version numbers for Professional are just +5 for the minor value from it’s community counterpart e.g.
Professional 220.127.116.11 = Community 18.104.22.168
Professional 22.214.171.124 = Community 126.96.36.199
So, if you make sure you downgrade to the equivalent community edition (or the next one in the sequence) in the first instance, I think you will have the least amount of problems.
Additionally, all the enterprise features that come with Professional are isolated in the code, so those folders can either be removed or simply ignored.
Like any Magento upgrade, you’ll just need to make sure you account for any changes in the design templates.
Hope that helps!