Whoops, misunderstood your question. My bad.
Yep, Access does not like changing a long interger to an autonumber even for an empty table.
If you have linked data in the existing table, I suppose you could add a new field, and give it an autonumber property.
Then you have to go through and reset your links to use the autonumber - a query should work.
Then make the new autonumber the primary key.
Richard