Its actually very nice, you can update your framework projects to 4.7 which can cross compile as dotnet core 2.1 then after you are comfortable with the dotnet core build you can drop framework.
I don't know if there's a good way to migrate the csprojs automatically (maybe https://github.com/hvanbakel/CsprojToVs2017?), but it shouldn't be that hard manually (by creating a new project), assuming you have just a few projects to migrate.