I've done multiple re-installs on three different PCs at home using ISOs I created from the Media Creation Utility. These installs have included release versions 1511, 1607 and 1703. Haven't tried it with branch 1709 yet though. Regardless of the machine, or Windows 10 release version, every one of them has activated automatically without issue. All three of these machines began life as Windows 7 Pro operating systems. In my gaming machine I've even changed SSD Arrays and reinstalled Windows 10 without issue.
The only horror story I've personally experienced was with a motherboard replacement of a co-worker. His upgrade of Windows 10 would not reactivate after changing the motherboard. He even went as far as calling MS and was essentially told he was SOL in this situation.
I guess a motherboard change is considered too big a change, requiring a new OS license?