well keeping everything seems flawless to me, i thought i might end up with some registry inconsistencies but no, everything is where it was, i ran ccleaner after installing win 10 and there was nothing to clean. had maybe 200 registry inconsistencies from before i installed the os.
my windows.old folder is probably over 10gb, i also still seem to have the BT and WS folders which are still hidden and combined are about the same. could delete those probably for an extra 20gb of space.
I did a force update at my dad's laptop and it didn't give me an option to keep everything. So I guess you can only keep everything if you wait for WU...