Recently we ended up adding a field to one application, in the USR layer and then adding it back to the ISV layer in another application.
The question was if we merged the ISV model into the 1st application, would the data still exist in the 1st application? Or would we have to create database scripts to restore this.
So off I went and created this flow which shows how I went about with my testing as per below.
Result: Success, There was no data loss. The fieldId stayed the same for the LIVE system in both scenarios.