Quote Originally Posted by Miranda View Post
Current: Shimano Ultergra SL 50/34
Considering: Shimano Ultergra 6700 or DuraAce 7900 53/39
There are exceptions, but in general, and specifically for the cranks you listed:
Compact cranks have 110mm bolt circle diameter.
Standard cranks have 130mm bolt circle diameter.
So yes, in general, you'd get a new crank.
Except... You can buy 39 and 53 tooth chain rings (from FSA I think?) in 110mm which will bolt onto your existing crank. However in my experience nothing beats the shifting performance of Shimano rings, and these are not available from Shimano. So my personal choice would be a new crank.

The new cranks you're considering are good. However with this current generation, Shimano's moved the two chainrings farther apart by about 1 millimeter, making those cranks tecnically incompatible with your current shift levers. I haven't tried it to see just what happens, but you can avoid the issue by using a crank of the last generation: Ultegra 6600 or Dura-Ace 7800.

With 6600 or 7800 cranks and a longer chain (maybe not; 3 teeth more means only 1.5 links longer and your current chain *might* be on the long side already?), you should be good to go as far as parts are concerned, only adjusting needed to get back on the road. Oh yeah, maybe a new front derailleur cable, as your FD will need to go 6mm higher to fit the new 53 tooth ring.

Have fun!