try using auto enter by calculation and check do not replace existing values. No calculation field.
As Rick says, you should use an autoenter.
If your field is right now a calc field, it WILL recalc if you change the formula. So start by changing its type to numeric, instead of calc. Filemaker is magic at this and will keep the value the field had before - try it on a copy if you are afraid. Then edit the field and make it autoenter - again Filemaker will be magic and offer you the former formula already in place for the autoenter.
You could add an "IF" or "CASE" to your calc, e.g. so that IF date > 2/27/2016 ( or perhaps IF Serial Number > 9568 ), Round 0, else Round 2
> I'm trying to update a rounding calculation from 2 decimal to 0 but I don't want my previous records to update. How would I do that?
Thank you Rick, siplus and gdurniak for your suggestion. I changed the field from calc to number and that worked. I tried the "IF" or "Case" suggestion as well and will keep it in mind for other use.
Thank you all once again, you guys are great.