Value of upgrading only 1 set of interconnects?

Answers will vary however I've experienced success with upgrading one pair of interconnects. If one has a good ear for music and an overall understanding of mixing and matching great success can be achieved. It's tunning the music to your preferences. With that said many people will suggest using the same brand interconnects etc.
It's all good!

BTW: which Cardas balanced cables are you currently using?

I've had good results changing just one pair. I fared better with keeping the best pair near the source.

If the sonic characteristics of the new cable are more to your liking you should get some noticeable improvement with one interconnect and more of the same with two.  I agree to start with the source and work your way down, although it's good to try in both positions just to see what you get.  Just trust your ears.  If that means two different brands of interconnects, so be it.  There's no absolute right or wrong here.  Best of luck. 

All cables work on the principle of subtraction. They all take something away, leaving the rest apparently more enhanced.

The real question is, do you want to change anything? If so, your room is always where you should start. Cables should come last.

Best,
E