Someone(r)(c)(tm)

I thought I couldn't change vehicle weapons either but later realised that you actually need to click on the label of the weapon and not its picture to change weapon. So if you want to change away from .30 Cal, actually click on the writing ".30 Cal Machinegun" and not the picture of the .30 Cal. Doing so should bring up a list of options (with rocket launcher being one of them). I thought it was bugged for quite a while until someone else suggested I click on the writing rather the picture. I can see how the picture and writing are supposed to be separate after having looked more closely at it but it isn't that obvious so might be better if clicking on the picture also brought up the list of options.