1) Windows 7 does not allow users to downgrade from more premium versions to less premium ones. One can only upgrade from lower versions to higher ones E.g. from pro to ultimate or hime premium to pro. So there is no official way of downgrading.
2) A hack to do an in-place downgrade is posted on the page below. Hack to In-Place “Downgrade” from Windows 7 Ultimate or Professional to Less Premium Editions My Digital Life
IDK whether it works for Enterprise.
3) IMO, a clean install is the best way to go. You get a fresh OS without any residue from the previous install. Windows OS, in any case, degrades in performance over a period of time due to installs and uninstalls of user apps. But you'll need to reinstall all your apps. That might take a bit of time but the end result will be worth it.