sql – How to ensure that future entries are assigned an ID that is +1 of the most recent ID, WITHOUT changing the IDs of older entries?

I am trying to move data (previously stored in an excel spreadsheet) into a new sql database.

This sql database will be used by mulitple people and it is important that users can add entries at the same time without any concern that the IDs will conflict.

This seems quite easy to do in a new database; simply use autoincrement.

Unfortunately, I need the OLD IDs to remain intact and unaltered by the new system. Anything I try to set up seems to edit the old IDs.

Is there a way that I can add this auto-increment functionality to a table, so that new entries are assigned sensible IDs, without enforcing that all entries already in the table adhear to this system?