I have not done this and I don’t know all the details but.
I would create an acf/save_post filter with a priority of 1 so that it runs before ACF saves the values. See details here https://www.advancedcustomfields.com/resources/acf-save_post/
In this filter I would somehow check the existing value against the submitted value and update the field as needed. This is where I don’t have the details. It would take a lot of testing to figure out how to accomplish it on both the front and back end.
When I was done I would unset any relevant field keys in $_POST[‘acf’]. Removing the submitted values will prevent ACF from updating those fields and undoing the work you did in the filter.
This is an issue when someone tries to edit both sides of a bidirectional relationship at the same time. I tell my clients not to do it. However, if there is no way to prevent this from happening then I’d have to work it out.
