Sadly, the Photos referenced file feature is just as poorly implemented as the one in iPhoto. iPhoto also couldn’t handle moving files to a new drive. It’s rather shocking.
One crazy idea – what if you name the new drive the same as the old drive, and copy the files to the exact same locations on the new drive, might that work?
I suggest doing this:
1) Copy the files from old to new
2) Eject the old
3) Rename the new drive = the old drive
Those steps are important otherwise, you might get a “-2” or something appended invisibly to the drive name.