As @nicholas__kerns has already warned you about, removing the screen on a Pixel 4a is already a semi-risky proposition; I've heard many stories of people breaking screens that started out intact before they tried to remove them, so having a cracked one just magnifies the risk of opening it up.
But that part aside, what you want to do isn't going to work. The problem is, the lower microphone is soldered to the motherboard so there isn't a separate part you can take out and just swap over. Here's a picture showing the location of the microphone; it's right next to the charging port on the logic board, marked in red.
[image|2739093]
Sorry, wish I had better news for you. If you feel you have the soldering skills to replace the microphone on the one that needs it, that's going to be your best option, assuming you can find the correct replacement part.