Once a given hardware has been updated to 10 and the 10 installation activated, it remains authorised for that hardware even after the July deadline.
(It has to be - because if you do a free upgrade with every intent on keeping it - but then for whatever reason have to do a factory reset, which will revert to shipping condition, you have to be able to re-do the upgrade to 10......MS don't want to stop you)
So yes, you can
(a) back up your 8 system
(b) update it to 10 before end July for free
(c) ensure 10 is activated
(d) revert to 8
(e) at a later date update the same system to 10 without cost.
FYI, though - there are some pretty good alternatives to MCE out there that will run on a 10 system, and (IMO at least) 10 is an improvement over 8.x