I was reading through this answer about the cosmic microwave background radiation, which implied that we receive it as microwaves because it's been shifted to that wavelength by the expanding universe. Did I understand that correctly?
If so, wouldn't that mean that the rate of expansion would change the wavelengths perceived? Based on our understanding, has the CMB ever not been microwaves? Will it eventually stop being microwaves? If/when it is visible light, would we be able to see it?
Yes you read the other answer correctly. The expansion does change the wavelength and not just the perceived wavelength. Each individual photon gets stretched as its traveling through a stretching spacetime. The CMB was released back when the entire universe was filled with a hot opaque plasma which emitted light as a black body radiation in a similar way as stars do. There was originally a lot of visible light. It will eventually cool off further than microwaves if the universe expansion continues which at this point most theories predict it will.
