There are a few approaches:
- Extract the metadata as XML, edit it with some scripting (Powershell, VB.net, C#, Python, etc), and upload it back
- Use a one-off extender rule, triggered via a Data Management job. The API function to modify relationships is SaveRelationshipInfo, search across the forums for examples.
- There are some Excel-based helpers for metadata management floating around in the wider community. I expect that sort of process might still be pretty painful over 1600 members...
Whatever you do, remember that data is stored against base members; so don't remove those, don't turn them into parents, and don't leave them orphan. Also keep an eye on constraints and channels, if you use them in combination with UDs.