HI, have a Dura Ace 10 speed 11-23 Cassette...
HI, have a Dura Ace 10 speed 11-23 Cassette on a 2006 Felt. Want to change to a Ultegra at least 12-27 but also saw an 11-28. I was told that in both cases I would need to change the chain and in the 11-28 I would also have to change the rear derailleur. Whats the consensus. Anything else I need to know
It is very difficult to say with out looking at the bike. If the chain was put on with the proper 1 extra link then it should be OK but with out seeing the bike I can't be sure. I would recommend ordering a new chain when you order the cassette, chains are relatively cheap and you probably need a new one with the new cassette.
As far as your R Der the Max R Sprocket of your 7800 is a 27t wile the 7900 R Der is a 28t. Your Der may stretch and make the throw but probably will not be quiet when running in the 28 but its hard to say for sure.