Yes, "interchangeable" as in "the copilot seat will physically mount in the pilot spot and vice-versa". But the recline levers will be jammed up against the sidewall and unusable if you do that, so if you're considering a replacement seat, you don't want to end up with two pilot seats or two copilot seats.
There are also minor differences in seats. I don't know if they were by model year or by purchased option or what, but for instance in my PA28 the pilot seat has a height adjustment mechanism, but the copilot seat does not. I've also seen seats with headrests and seats without, etc.
Bottom line is, all PA28 seats mounts might fit all PA28 seat tracks, but the seats themselves are not all identical.