I just did this to some sweater sleeves. It was not as easy as I thought it would be--it was a k4, p1 rib, and the yarn didn't want to unravel at the purls. I think it would have been easier if it was just knit stitches, though.
What I did was pull a tapestry needle with yarn through one loop along a row of stitches to hold those. I tried to unravel, but since it was so difficult, I cut off most of the remaining knitting and pretty much picked out the rest. Then I inserted a needle and got all the stitches oriented in the right direction, making sure they all were on the same row. They weren't, so I had to slip some and tink some until it was all even.
Once I got all the 'extra' stitches off, knitting the other direction was no problem. I took off a straight portion that was too long in favor of ribbing, so the difference wouldn't have shown in that case, anyway.
I think that once you get a row of stitches on a needle and knit in the opposite direction, it wouldn't really show, anyway.