Sorry it's not much help but I've asked the same question, I have a 300tdi Disco and have been offered a 200 pump !
Three respected Land rover mechanics have told me NO they won't interchange.
Two equally respected folks have said YES they will, one has done it, putting a 200 into a 300, but it felt "under powered".Your going 200 - 300 so that may give you more power ?
I guess they will swap but need some kind of tinkering with !
Not being a great DIY mechanic I've decided to wait and fit the right pump...so if you have a 300 pump spare to sell I'm interested ?