Change Derailleur on CX Bike I have a Giant 2013 TCX Advanced. It has a Rotor 3DF, 36/46 Crankset and a SRAM XG1190 11x28 cassette. I'd like to swap the cassette for an 11-40 or 11-42.

The guys at Wolftooth said: "..derailleur does NOT have enough capacity to handle an 11-40 cassette and a double chainring. You will have to be careful to not use small/small and large/large gearing combinations.."

Can I simply buy a new long cage derailleur, such as Sram Unisex's NX 1x11?



Edit: 

Thanks David and..Ouch! Never realised that! So, to keep it relatively simple (and less expensive): my best choice to get to at least 1:1 is to replace the 36 crankset with, say, a 32 and the cassette with an 11-32? ...Again using the roadlink from Wolftooth? Do I need to replace the short cage? Can I replace the 36 with a 30... (and install the 11-32 cassette?).
You can't just get a derailleur and cassette to make this work the way you want. The components are designed as part of a system with parts that are designed to work together, at least according to SRAM.

You will want a rear derailleur that matches your SRAM Red shift levers.
According to page 2 of this chart:
You can get gearing of 11-32 or 12-32 with a range of front chain ring sizes if you use the mid length arm of the SRAM Red 22 rear derailleur. This would be a derailleur and cassette purchase (I'm assuming your bike came with a short arm rear derailleur) that would get you closer to your goal, but not all the way.

To get to 11-40 or 11-42 and have all the parts match you'd have to go with a different parts group.
According to page 3 of the chart linked above the Force 1 or Rival 1 shifter, long cage derailleur and cassette will go to 42.
You'll also need a longer chain.