I used to worry too that classic clothing could look frumpy, but after reading some of Angie's recent blog posts - I realized that classic pieces in unusual colours was a fun way to make sure it was "modern classic" and not "frumpy classic."
I've also noticed too, that sometimes it's all in the accessories - fab shoes, an amazing handbag, or a stylish and well-selected pair of eyeglasses. Even watches...I haven't updated mine, but I can see how all of these would keep the classics modern-looking.
In the meantime, I don't think you look frumpy at all! It sounds more like it's a shift from junior styles to misses, and I don't think this equals "frumpy" in any way.
ETA: some examples of Angie's Modern Classic or Classic Modern outfits:
http://youlookfab.com/2012/02/.....eek-day-4/
http://youlookfab.com/2012/01/.....-new-year/
http://youlookfab.com/2011/11/.....lue-dress/
http://youlookfab.com/2011/09/.....nd-simple/
There are lots more, but as you can see - these outfits are put together with classic pieces. Amazing, isn't it?