You don't have to convert them over if you don't want to. The old format works just fine with Visual Studio 2017. The new format is cleaner, though.
You could also try this tool: https://github.com/hvanbakel/CsprojToVs2017
We've used that here with some success, but we also had to do a fair amount of manual cleanup after doing the conversion.
Generally we do as you are doing already - comparing an existing good project file with one we are converting.