Can I install Win 10 over Win 7 without losing my data?
I am not sure about this since i'm not using windows, but i
believe there is an "upgrade" option which keeps the data.
Even if there is not, you could just copy all of your data (blocks, chainstate, wallet, etc..) to a hard drive and then copy it back after installing Win 10. Then you don't need to downloaded and process the whole blockchain again.
), "upgrade" option is definitely exist. But some people experience problem during/after upgrade process, so it's strongly recommended to backup all your data before doing upgrade process.