I recently installed a new dp and exhaust. Texas (each state is different) has confusing laws on the subject. I read them and discussed them with an inspection station. Apparently, here anyway, if it has one, you are supposed to keep it - one law says it is unlawful to remove an existing one. BUT, diesels are not required to have one so it is not visually inspected. I had my truck inspected a couple of weeks after the upgrade and it passed.
The upshot is, if you have one, dont remove it but if you do noone is likely to know. As for federal law changes, each vehicle is only held to the standard in place at its time of manufacture. So, even if the standards become more restrictive next week, we dont have anything to be concerned with.
For 1997, Ford was the only domestic diesel truck maker that put one on. Or so I understand.