Replace rear deraileur without breaking the chain I currently have a Shimano Dura Ace 10 speed rear derailleur with a 12-25 cassette. For some hilly rides I'd like to change to a 12-30 cassette. 

The maximum the rear derailleur can handle is 28, so I'm thinking of getting a Shimano 105 RD. I'd like to be able to switch between these two set-ups but fear that breaking the chain each time is not a good thing to do.

Is it possible to change the RD without breaking the chain by taking apart the pulley wheels?

Also is it possible to fit a 12-30 cassette on a Shimano Dura Ace 10 speed short cage RD?
You're likely to destroy the derailleur by doing that too many times. 

KMC and Wipperman both have 10 speed quick links which are a better thing to do. Note that you'll need a chain big enough for the larger cassette (or, with a quick link have 2 chains).