Actually that's only a 2" setback (as the centred stance would have them at 19 3/4" tip and tail)... That's pretty mild. I think my board comes with at least that on the reference inserts!
You could go another set of holes back for tomorrow. If they have tools at the hill you could always do a couple runs and swap them back further and give it a try. Only way to really feel a difference in setup is to do it during the day so conditions aren't a variable!
If you do, just make sure you either melt all the snow off and dry the inserts out prior to swapping, or scrape all the snow off and blow into the inserts to melt any snow and hopefully push the water out. Breath is warm enough to melt ice in inserts even out on the hill!