A question about modifying Character Sheets

If a character sheet is modified to add additional fields, does it reset any already filled out character sheets?

What do you mean by "modified"? Do you mean submitting modifications to a sheet on the official GitHub? If that's the case, then no, it will not reset any existing sheets. Or rather, it won't do anything to any existing attributes on a character, and any attributes that match the new version of the sheet will be used as appropriate. If all you do is add fields to the sheet, players already using the sheet will be unaffected (except that they get new fields to put information into). If you remove fields, the information will still be on existing characters, it just won't be editable from the character sheet tab.

My DM was considering adding a couple fields to the D&D3.5 Character Sheet we've been using to get some more accurate calculations of AC's, among other things. However, there was concern that making any changes might clear all the work everyone has put into their character sheet macros.

I'm going to move this to the Character Sheet forum.

If they modify the original sheet's html and css and use a custom sheet with that modified code then all the original data will still be in the new sheets.