Convert Shimano Ultegra 6870 rear mech from short cage to medium cage I would like to ride with a 11-32 cassette but I have a short caged Ultegra 6870 derailleur. Can I just swap the cage? Or are there other differences between the derailleurs?

Cheers
In short, yes. 

A single web search for "Ultegra 6870 derailleur spare parts" will find an exploded view diagram with spare part codes. It is fairly clear that the only differing parts are cage plates. The parts go by codes Y5YC98080 and Y5YC25000.