My BIGest frustration with V11, is yes, V 10's get "locked out" from a return to Version 10. I thought the flatten surfaces would be my big purchase with V11, but with my total file loss, ( I had converted MANY files to DXF for the Version 5-7 ( 2 different engines, but V2 and 7+ are OK together, I've reverted to 12 year old files on 9 projects! One other thing I will mention of the High Sierra: ya notice how long it takes to "grind" a secure empty trash? =(an old TCP program would take a few hours. now, High Sierra, it's seconds! However, the initial engines take a longer response time- some splines etc.= and other surface/ solid renders, take 20 minutes!
Go with Mitch's mention with a 'clean install, and if you get confused, use "clean install" for the search here, as it has become minimal since Version 10. I take it, this file is purchased?