I had to do this recently and the steps are listed below
Export the users from source workspace
Export the users from the target workspace
Create an import file from 1. of the users that were not already in list 2.
Import the model to the target workspace
The model rights all imported correctly for existing users
Import the “new” users from the file created in 3.
Repeat step 2. Cross check the access rights against 1.
Amend the workspace settings for existing users, because those settings do not come across from step 4.
The access rights for existing models in the target workspace should be unaffected and any new users should be set to No Access
Hope this helps