I know that Thor makes pants that can convert into shorts. They call them, "Static" pants.
I had a pair of them and they were fine other than the fact that they had a clip (where you have to squeeze the 2 ears together to get them undone) that held the fly shut instead of a button or hook and loop setup and it was often very difficult to get them unclipped
I think Fox makes pants like that too and Id bet some other companies do as well, but Im not 100% certain.
Just keep in mind that you cant wear shorts when you are competing in an AMA event, you will have to put the legs back on them.
I also found that my right leg would get really hot because there was pretty much nothing protecting it from the head of the exhaust.
Have you considered a set of vented riding gear, such as the Thor "AC"? That might be something youd like to consider as well.