There is a couple of parameters we want to attract your attention to.
Overwrite behavior
If your AutoNumber is set once and for all, you want to make sure it is set to Never overwrite so that it does not get overwritten when running AutoNumbers for the current lookup.
Prompt screens
It might be a good idea to check these options. First it gives you a visual clue that an AutoNumber is going to be issued and allows you to cancel it, in case this was inappropriate.
Second it gives you the ability to edit the AutoNumber in case it does not look like expected.