My understanding is you want to permenently remove the autonumber column for a table. If this is so, you can crete a "Make Table Query" that includes all the current columns except the autonumbered column. Running that query will copy the curent table to a new table without the autonumber field. then rename the new table to the old name create your new index your all set.
Now if there are related tables that use the exsiting autonumber as a reference this still is not a real problem. use a cstr() function on the autonumber field when making the make table query and then later on reconvert it back to a number so that you can maintain your links off of those numbers.
Good Luck
ssecca