Yup still holding out hope that they will build in a main menu background switcher before they end development. Currently the only way to see these other ones is to disable either FG or AoM etc. depending on what you want.
This can be done one of 3 ways:
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Use my tool here to easily play any version you want with a click:
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Manually edit your launch options each time you want to switch:
Play with friends although not everybody has the expansion(s)
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Or simply uninstall the expansions. Which of course involves redownloading them every time you want to play them again.
Keep in mind that your characters are not backwards compatible. An FG character cannot play in AoM or Vanilla nor will your FG characters even be visible in the menus. You will have to create new characters.
Be mindful that if you do make these new characters and then switch back to FG these newly made characters will indeed be visible in the menu. Here they are safe but if you make the mistake of entering the game world with them they will then be converted to FG characters and would no longer be able to go back. So be attentive.
If instead you are more referring to the loading screens themselves you can mod them to be whatever you want… but you need to do a little bit of work on your own.