What for are you using an auto-increment field? Field Position looks like something user would see and for that using a database auto-increment does not seem to be the right solution. Auto-increment field will always get the next value from the database (always higher) and should be only used for integer based primary keys (which we do not recommend anyway, that’s why we use Guids).
The user interface will do it for you automatically when set as OrderMember. Up/Down arrows will change the current order and inserting a new row by + will increment the number.