The Insider’s Preview for Windows 10 (which is in constant development) announced that build 10565 will now accept valid product keys for Windows 7/8/8.1 to activate. According to Microsoft, the key will have to correspond to the edition of Windows 10 that you are activating. Meaning that if you have a key for Home Edition, you cannot activate the Professional Edition.
The only downside is that once you activate, the activation remains with your device permanently. That means that once the upgrade offer has expired, you can no longer use the old keys to activate any system but the original (which has always been the plan).
Frequently asked questions have been addressed by the software behemoth, so go ahead and don’t take my word for it.