I tried yesterday and today. (I have to boot 8.1 specially just to find out)
Just get the "we'll let you know when" message via WUD.
My W7 isn't eligible, not that I'd allow that to be downgraded anyway, but booting 8.1 last night screwed with my W7 boot again despite having had fast start disabled for years.
Took about 30 minutes to get past chkdsk scanning every bl**dy one of my >5Tb's of HDD this morning.
just as I posted the above, got my "you've got mail" tune (Clapton - Layla - first bar), to find Gabe Aul just sent me a note to tell me my "valued insider" status entitles me to get it now.
I'll try again after I shut down W7.
Writing this from W10
WUD has still been doing "failed" attempts when I left 8.1 running overnight (80240020) but a little delving around revealed that I've had the $Windows.~BT hidden download file on the system all week.
I can understand MS saying "wait, we've got a huge queue waiting to download", but I completely fail to see why they said "wait, you've got it all, but we're still not going to let you start the upgrade yet"
Turns out a simple "EnableOSUpgrade" key inserted in the registry was all that was needed, and the previously downloaded 4Gb file switched on and changed 8.1 into 10 in next to no time.
Don't understand the thinking behind that at all !