Well, since you ignored the post about being within ArcMap, I'm going to have to assume that when you say Field Calculator, you actually mean the calculateField python tool meaning that you want to do the entire thing in code without having to open ArcMap.
One option would be to run the
ConvertCoordinateNotation_management (in_table, out_featureclass, x_field, y_field, input_coordinate_format, output_coordinate_format, {id_field}, {spatial_reference})
tool using your OID (or other unique field in your data) as the id_field (which is specifically included to allow for joins back to the original table). As you said, this would create a new FC, but could be saved in_memory to increase speed.
Then makeFeatureLayer on your original table so that you can do one of the following (won't work direclty on the FC):
Now, if you want to add these new coordinates to the existing table, use the joinfield tool to add the new columns to the original table.
Or, if you want to "update" some existing fields in the original FC, you would use addJoin to join the two together, then caclulateField and calculate the existing field(s) to the newly joined fields, then remove the join and you are done.
R_
