I would keep the same HUD normally unless I switch it out for debugging purposes.
Sometimes for a level like the main menu you will be in UI mode all the time and what is on screen will be so simple you can get away with just a default HUD and the main menu UserWidget on it, but usually you make one HUD class for the rest of the game and inject on it what you need. the HUD just implements core functionality. You could always decide what UserWidget classes to spawn inside of it based on what level you are in. For widget it is common you write core functionality once in c++ and let the designer make blueprint widgets based on them where only the design is implemented. then you can simply spawn a widget of design X or Y through the HUD. I let the HUD keep track of what is on screen.
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