Should I switch from the MGIB to the Post-9/11-while on active duty?

You can do it when you get out. I didn't switch mine over until after I had been medically retired. Look into both programs though; depending on where you go to school, the MGIB may be better (i.e. my friend was from TX and he automatically got his school paid for in TX so th MGIB just gave him money in pocket whereas the P9/11 would have only given him the stipend). So your homework prior to switching over.

My 2 cents is either do it while on Active Duty and or get all the names and phone numbers you need before you depart service.

There is no switching. You simply choose which one you will use when you decide to draw benefits. This is one of the more common questions I get in my office.
There are other things to consider. You can use all of your MGIB and then request 12 more months of the 9/11. MGIB pays to you and may cover programs the 9/11 does not. Once you use all your 9/11 you may get your $1,200 refunded to you. Once you elect 9/11 you may not revert to MGIB. If you transfer your benefits to your dependents that will change it to 9/11.