I recently had my Giant OCR tuned and the shop recommended that I replace the chain and the rear cassette as the chain had stretched. After this repair the chain now slips off of the middle chain ring to the granny ring when I am in the lowest rear gear. The shop is now suggesting that while they don't see anything wrong with the middle chain ring that I replace this as well as sometimes new chains do not mesh well with old parts. He is also suggesting that I replace my bottom bracket with a shorter one to change the chain angle. Does this sound reasonable?