Just completed an upgrade from MODx Version 2.0.6 pl to 2.2.14 pl.
(I didn't go to 2.1 first, just straight to 2.2.14)
All went smoothly apart from some minor URI resource errors.
Reinstalled Tiny MCE, turned Friendly URL's off then on again, cleared and deleted files from core/cache. All Fixed.
Ran Bob Ray's excellent
Sitecheck CMP component -
http://bobsguides.com/sitecheck-tutorial.html - 100% no errors.
Well done to the MODx Upgrade Team who put this together. Painless and stable.
Many thanks for your excellent efforts.