If it is colour neg then a couple of stops of 'overexposure' usually doesn't matter - in fact it lowers the graininess. I use Portra 800 in my carry-all-the-time camera because I know that I can set the meter to anywhere between 200 and 800 on the same roll and get good results. I usually set the meter to 400 to get good, low graininess results.
Remember that the ISO speed of colour film is based on the minimum exposure that the film needs, and that there is plenty of dynamic range over that.
It's easy enough to switch films, as already explained, so that is preferable if you know that you want to shoot a lower speed film.
Best,
Helen