You could copy the Library to the hard drive and then put the original Library, which by default will be in the Movies folder, into the trash.
DO NOT DELETE THE TRASH at this moment.
Check that the transferred Library is working.
If it works OK it should be safe to delete the trash to free up space.
Do you actually need to keep the old Library and Projects? Usually the project is exported (shared) as a video and the Project is deleted.
The only reason for keeping old projects is if there is a chance you will want to re-edit them in the future.
These are things that only you can decide.