They're not identical, the af/s is silent wave focussing, the af/d is cam focussing.
It's not a straight swap, the af/s is no longer manufactured, and that doesn't even make them cheap, even if you could find one.
I would think you'd be looking at the thick end of 6-£700 for a secondhand 80-200 af/s, the af/d could be bought new for that.
The bang for buck lies with the af/d, the af isn't as fast as either the 80-200 af/s or the 70-200 af/s vr, but it isn't pedestrian.
Both the 80-200's are legendary, I'm sure the 70-200 will also become so.
Make sure you know which af/d you are looking at, some of the older models
do have slow af.
80-200 af/d ed
80-200 af/s
70-200 af/s vr
easiest way to tell the difference between an af/d and the af/s at a glance is the af lock buttons on the side of the af/s between the grip rings.