Replace the draft inducer blower with a model of a slightly different speed

Does the ratio of static pressures between two stories differ at different blower speeds? That's a very interesting question, but I believe it's a moot point. Any dramatic difference would mean your system isn't balanced correctly in the first place. (needs zoning, split systems, or an actual balancing)

Stack effect will cause plenty of heat to go upstairs, while the cold air will settle. I'd be more concerned that it won't cool right. Except that furnaces usually only run at high speed for AC because cold air is denser, making it harder to push. If your old system heats and cools just fine I don't foresee a problem.

Without being in the correct centralized location (which is somewhere on the first floor of a two story), the thermostat will not control the house properly. In low-fire mode, indeed less air will go upstairs and downstairs, per minute. Blowing softly is what makes it more comfortable. It will run longer and more often, reducing the intermittent temperature differential, keeping the house nice and even.

So no, the same amount of 'less' air will be coming out below as above. Your contractor either is trying to up-sell you into a split system (or knows more than I do) or convince you that zoning is required (a possibility, however this can be retrofitted later at your discretion). Either of these additions would dramatically increase the cost.

Unless your house has automatic zoning and was balanced by a genius, it will always be hotter on the second floor in the winter, and colder on the first in the summer, just like everyone else's.

There should be a schematic printed on a sticker on the motor. Without knowing the make and model, or looking at the diagram. I'm guessing based on other motors.

One common "standard" is as follows:

• White = grounded (neutral) (clockwise).
• Black = High speed.
• Blue = Medium speed (medium low).
• Red = Low speed.
• Brown = Run capacitor.
• Brown w/ white = Run capacitor.
• Green = Ground.
• Yellow = Medium speed.
• Purple = Grounded (neutral) (anticlockwise).

Optional wiring available on some motors.

However, without the make and model, or the schematic, this is just a guess.

Update

The two brown wires (Br and Br/Wh) should be connected to the capacitor. If you don't have one, you may need to install one. The brown wire from the old motor should have been connected to it, so try to remember where you removed the brown wire from.

The white wire from the motor, should connect to the white wire in the junction box. This is L2/neutral, and completes the circuit.

The remaining wires are for selecting the motor speed (Blk, Blue, Red), so you're going to have to figure out which speed you should be using. If you can remember which color wire was connected to the red wire in the junction box, that should be the same color wire to hook back up. Typically the blower is run at low speed when in heating mode, but that's no guarantee that your equipment was set up that way.

I wasn't able to locate any documentation on your equipment, and without a schematic I can't be any more specific. If you could include a good photo of the schematic, I could show you where to make the connections. You should be able to find a schematic in the manufacturer's documentation, or on the furnaces access panel.