The ability to switch sides on your armbar quickly and while maintain a tight position is very important.
In this video I show one of the ways to dynamically switch directions for your opposite side armbar. This is something that we practice during our drills class and can really surprise your opponent.
I use this any time my opponent buries his/her arm and I cant seem to finish the current armbar. Remember that you always rotate over the body and not over the head.
The first step is going to be to control that far arm (reference the video). By controlling that far arm it will ensure that your opponent can not recross his/her arms to defend on the far side. After you've controlled the far arm you can take your other hand and post it on the floor, that will allow you to push off and come up for the rotation. If anything goes wrong in this transition you should still be ok because you'll stay on top.
This is NOT something that you're going to be able to do without practicing. I highly recommend drilling this to be able to use it effectively. I can tell you that once you have refined this transition that it is HIGHLY effective.