I started out the Insiders tech previews with a clean install from the beginning, and have upgraded every build since 9926. I never upgraded from W7, although ShowKey says there is a W7 marker in my firmware. I will wait and see what happens after the 29th. Could be, that MS will let you enter your W7 key and activate based on that. If not, you could restore an image of W7. Or, you could just remain as an Insider.
I'm afraid Flash may hand around for quite some time, too much stuff is in Flash and all ether has to be converted (doubt that) and new stuff made in HTML5 or whatever.
my laptop is virtually useless after the new patch
no internet, can't open chrome, task manager reports 62% CPU usage
but gives no idea what is using that ..
trying to get back to previous build
or anything to get system stable
the windows update are when you can uninstall build
is unavailable .. damnnnn