'''96 Litespeed Catalyst, '05 Litespeed Firenze, '06 Litespeed Tuscany, '12 Surly Pacer, All are 3x8,9 or 10. It is hilly around here!
To change the rear derailleur without breaking the chain, remove only the idler (lower) pulley. You may have to "scissor" the inner and outer cage plates apart a bit to get the chain through the locator tabs and that will loosen the pivot bolt for the jockey (upper) pulley but it isn't necessary to remove it.
Install the new derailleur, thread the chain through it, replace the idler pulley and be sure the pivot bolts on both pulleys are snug. Adjust as required.