Like me you have two libraries for space reasons (although for me it is iCloud space).
There is no easy built in way to copy images from one library to another. You can only export from one, and import to the other.
The problem with this - is you can either transfer your edited versions. Or your original versions, but not both. Actually you can do both as two steps, but then they will appear as duplicates of each other in the destination library.
The only acceptable (for me) solution is to use the app "Power photos" which can easily transfer images from one library to another, maintaining both edited and original versions, and nearly all orgnisation, keywords, and metadata. The only thing it cannot transfer is face/people data.
It also has other functionality (such as duplicate detection) - however, it is a paid app.
See:
https://www.fatcatsoftware.com/powerphotos/