Upgrading win7 to win10 isnt much of an issue only if the win7 is healthy for the upgrade, user that had adware/virus/malware, stardock, accounting software & custom software would have more issue with an upgrade than a clean install.

Licensing isnt much of an issue to most user, afaik even those invalid licensed copy of win7/8 during the free upgrade period could be upgraded as a qualifying license copy. so the only issue are hardware compatibility & how healthy is the previous OS before upgrade.