There are no real "pre-activated" iso's, the activation probably is done by setupcomplete or autounattend.xml Code: Check for an XML file at the root of the iso. Check the ISO for a $OEM$ folder. Open the install.wim/esd with 7zip and browse to "ISO\sources\windows\setup\scripts" to see if there is something in it. Tip: When it's a homebrew release that uses re-loader, i would not touch it at all.
RemoveWAT also patches/replaces activation files (i.e. slmgr.vbs to fake activation when asked), therefor WATFix was created.
Seems that the best and easiest solution to this quandary is to simply download a fresh ISO directly from MS.