What is the difference between the 7 or 8 speed rear der guide pulley and the 9 speed rear der guide pulley? One difference is the 7 or 8 speed guide pulley is 10 tooth and the 9 speed is 11 tooth. Are they interchangeable?

Edit: Sorry... Shimano Ultegra rear der. I want to put 9 speed pulleys on 7 speed der.

You may want to specify what brand, year, and model of derailleur you are talking about. There are only about 10,000 possibilities.

But just to toss out some random maybe-related info, Shimano 7 and 8 speed guide pulleys have no problems with 9 speed chains or drives. Whether or not they are interchangable for replacement purposes is unknown from your post, though I wouldn't count on parts that are 5 years apart having many interchangable parts.

peace,
sam

If they fit, do it.

Exactly!

And if they don't fit, probably nothing will explode.