Retrofit assemblies and an HVAC unit including a retrofit assembly are disclosed herein. In one embodiment, the retrofit assembly includes: (1) a base plate, that is mountable to a face of a motor speed driver (MSD), having an attachment area, (2) a standoff bracket that is mountable to the attachment area of the base plate and having a configuration that secures the MSD in a position relative to an interior wall of the HVAC system and provides clearance between the interior wall of the HVAC system and the MSD such that wires can be routed between the MSD and the interior wall and (3) a control interface, electrically connectable between the MSD and a thermostat of the HVAC system, configured to differentiate thermostat calls from the thermostat and translate the thermostat calls to coordinate blower speeds for the blower motor via the MSD.