First off I recommend upgrading to 10. It runs better especially on lower ram systems, and there is definitely improvements in app performance and startup.
That being said, these are the issues I've run into upgrading users to 10
1) Update your BIOS! Especially if its a Laptop: There are issues with the screen going black after updating to 10 that are caused by an outdated BIOS, especially on older Dell Vostro and some ASUS systems. If you forget chances are you can get a screen by plugging in a monitor but update the BIOS to avoid it in the first place.
2) if you have IDT audio installed on your PC, remove the driver NOW from programs and features!: Older Versions of this driver is FUBARed and will cause explorer to crash infinitely while it is migrating your profile, which will screw it up to the point where you will have to rollback to 7. Cheap Dells, HP's and Toshiba's usually have this sound card. 10 Will detect it as a HD audio codec and will work fine without the crashing, or use the latest 8.1 driver if your manufacturer actually has one. Not sure why MS doesn't detect this as incompatible during its check.
3) Windows update on windows 7 will screw you out of time: Windows update is so FUBARed on 7. it alone is a reason to upgrade to 10. I've found the best way to update from 7 is to install windows 10 setup to a flash drive, keep the upgrading system off the network until you see the installing windows circle, and open a administrative command prompt and "net stop wuauserv" about every 5 minutes during install. This will cut load times down significantly cause the install process will start a windows update session every 5 minutes and with win7 taking about 2 hours a checkin the above will get 10 installed in minutes rather than hours
4) Profiles may not migrate on first run and run temporary profile: Seems to happen to a lot of 2010-2011 HP units for some reason. Boot with safe mode and the profile should fix itself although the start menu will be clean of Squares.
5) Wifi card goes AWOL after Shutdown/Sleep: See this one constantly with 2013-2015 Dell laptops with a dell customized broadcom driver. Some people say removing the Dell driver altogether and using the 10 driver works but YMMV also the latest Dell driver does not fix this issue. I typically disable fast startup and Wifi Power saving options to get around this one but it will still happen from time to time.
6) Start menu and/or Windows store/control panel disappears or will not load: running tweaking.com all in one repair with all checks enabled and leave the system on overnight seems to fix this one. also reseting default apps may get this one fixed. if it doesn't fix after doing the above than use your win10 Stick to reinstall Win10 as an upgrade install.