That is actually how its done. When you first import the object into your slicer BStudio, you assign it a material to print with. Those settings from the material are the ones your printer will use to print. If you set everything as PLA you wont be able to print in petg from your AMS. Same goes for external.
The only way you would be able to print would be, to trick your AMS into thinking it has PLA instead of PETG in the slot.
Which is silly to begin with.
The way to do this is cancelling the print window, going to the Device tab and assigning the right filaments and then print. Unless the print started you are able to change filaments unload and load, change colours etc. once the print started or the printer is busy you have to cancel or wait.