I'm using the new yarn I got from Uncommon Threads. The colour is not a bad match: Old yarn on bottom, new yarn on top.
I actually had to angle the knitting in the light to show the colour difference, but there is quite a big difference in the texture of the yarn. I dont know if Rowan changed the base yarn for Cotton Glace in the last 15 years, or the original yarn 'softened' while stored. The old yarn is considerably softer and seems thicker than the new yarn. I'm hoping that a wash once the cardigan is finished will hide some of this difference. Otherwise I will need to keep moving my arms to disguise the effect.