How to switch firmware from 12 V heated bed to 110 V SSR heated bed?
How does Marlin know that the heated bed pins are controlling a relay now instead of a bed directly, and therefore should limit their current output?


Marlin does not (need to) know this. Simply put, all Marlin does is switch on/off the voltage at the heated bed output. The amount of current that will flow is a function of the voltage and the device connected. If you connect a heated bed to a 12&nbsp;V voltage, a high current will flow. If you connect your relay to 12&nbsp;V, only a small current will flow.


  In other words- I am worried that if I just drop in the relay, it will burn up since the board still thinks it needs to supply high current to the bed.


You do not need to worry about this. Your question is essentially equivalent to this: "if I replace my 100W incandescent lightbulb with a 1W LED one, do I need to replace the light switch in the wall?" The switch (i.e., your printer board/Marlin) does not care what load is connected to it, so long as it does not exceed the maximum rated current.