HI @scferg
I would use the acf/save_post filter, according to the docs:
This action allows you to hook in before or after the $_POST data has been saved, making it useful to perform additional functionality when updating a post or other WP object.
If you’re trying to update a non-repeater field, then use update_field();
You could also look at acf/update_value I wonder if this may help more with the comparison side of things.