.net 4.8 projects cannot reference .net core libraries, nor can .net core call 4.8 project libraries. This means to migrate to .net 8 you need to update every project in the solution to .net 8.
an alternative is is to update all libraries to .net standard 2.0. These are callable from both .net 4.8 and .net 8. This can be done a project at a time. When all libraries have been migrated, the the main project(s) can be migrated to .net 8