Door switch configuration - Donga switches marking

Microwaves typically use both NO and NC switches. Mechanically they are very similar, and as you've seen, you can differentiate them by the position of the terminals. So no, they won't reverse their function when they fail; what'll happen is they either won't open or won't close no matter which style they are, or they can be slow to operate when they get corrosion or contamination, or the resistance can be too high when they're closed; these are all common failure modes for them.

Those switches are cheap enough that I'd suggest you just go ahead and replace them all. One pro tip I figured out is that you can buy switches that have both functions, NO and NC and use those for either. This kind will have three terminals instead of just two, so when you go to wire them up, you just have to be careful to put the wires on the two terminals that were used on the old switch.

On the ones I'm familiar with, one terminal is labelled COM; that'll be the same for either NO or NC. If you look closely at this picture, you can see which terminal is NO, which is NC, and which is COM.

On NC or NO-only switches, they just have the COM terminal and the one corresponding to their function, NO or NC. On this style, you can use the NO or NC function by just plugging the wire into the correct terminal. I've seen some that have both connectors molded into a single plug; in that case all you have to do is just plug it in, and it'll be in the right position.

Anyway, I'd go ahead and buy some new switches; if you prefer you can get yourself two NC and one NO, but as I said, it's a lot easier just to get three NO/NC switches and you can use them for either function.

Short of that, you could try cleaning them with something like electronics contact cleaner or even brake cleaner; anything that will act as a solvent and evaporate completely. I've had microwaves where the switches just got gummed up and their action ended up being delayed, causing it to blow fuses. Cleaning helped in that case, but just to be safe I replaced them anyway.