Can I retrofit a 2-speed or variable speed blower on a Payne PG8UAA forced air furnace?
According to the manufacturer's documentation, your furnace should have a 3 or 4 speed blower motor. The problem is, the furnace control module doesn't use a separate speed when the unit is running in continuous FAN mode. 


  When the R-G circuit is made, the blower motor operates on heating speed.


The control module only uses two speeds, one for COOL, and one for HEAT/FAN.


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You could run the blower at a slower speed during both HEAT and FAN calls. But you'd have to insure that the blower is maintaining adequate flow during heating, otherwise you will damage the system. This is not recommended, as you could damage the system, and void the warranty.

You might be able to swap out the control module, but you'd have to contact the manufacturer to determine if there's a compatible replacement.

The other option is to build your own control circuitry, but that's beyond the scope of this site. And will likely void the warranty on the unit.