I should mention that I have never exactly tried this technique, but it should work. :)
First of all, the technique does depend on all the record ID's below 1600 existing. If you ever deleted a record from your test app, then that will be a showstopper.
So odds are very high that you will need to copy the app without data.
So I think that the steps would be
1. Copy the app without data
2. In the Copy app change the key field of the main parent table back to [Record ID#]
2. Then create say 2000 records in the main Parent table from excel by importing say an X into a dummy field. That will run the Record ID# up to 2000.
3. Next is to copy the data for the main Parent table across to the Copy App. You can actually do a table to table copy now from your old app into the new app for the main table. Just run a "List All" report from the old app and use the "More" button. The fields should all line up properly as they are identically named, but then carefully map the Legacy Primary Key into the Record ID#. It should give you a warning that existing records may be updated. That is exactly what you want to happen.
4. If that all works, then you will similarly do a List All report from the child table and carefully map the field for [Related Primary Key] into the field [Relate Parent] but in fact I think it will line that up correctly automatically as really it is still being mapped to [related parent].
5. Repeat for the next child table and then of course you may have some other tables with data that do not have relationships.
