"They've been telling me that the previous furnace would have been burning at a hotter pace than the new ones and that may be contributing to the difference that we're experiencing."

This statement makes me think they don't know what they are talking about or that they're blowing smoke at you. If they messed with the ductwork, then that's the first place I would start.

@Sktn77a thanks for your comment.
My instict is telling that I should be after them about sizing that plenum back to the original size.

The main floor and upper floor of a two-story home will typically have different heating and cooling loads--even if the living areas are equal. Trying to maintain the same temperature on both levels with a single thermostat won't work well. In fact, a 3 F temperature differential between levels is not too bad for that configuration.

I think your best bet is to install balancing dampers in the duct work which will allow you to partition the air flow between the two levels.

"The main floor and upper floor of a two-story home will typically have different heating and cooling loads--even if the living areas are equal. Trying to maintain the same temperature on both levels with a single thermostat won't work well."

This is true in general but if this was not happening before the replacement, they did something else when they replaced the equipment.

This is my point as well, that there was not a problem getting air to the upstairs prior to their vertical plenum work and equipment replacement.

As mentioned I was having to hold the back to the 2nd floor.

I'll be sending a camera down the return duct from thr 2nd floor tomorrow and will update with what I discover if anything.

Thanks

"This is true in general but if this was not happening before the replacement, they did something else when they replaced the equipment."

It's pretty clear the replacement contractor "did something else when they replaced the equipment." Indeed, based on the information provided by the OP, the replacement contractor 1.) reduced the heat output of the furnace by 2%, 2.) reduced the airflow by 6%, 3.) improved the combustion efficiency by 22%, and 4.) modified the ductwork.

The OP needed to restrict the air flow to the 2nd floor prior to the equipment replacement and needs to restrict the flow to the 1st floor now. I see that as a six of one, half dozen of the other issue and a reasonable trade off for the increase in combustion efficiency.

Attempting to control two levels utilizing a thermostat located on one level doesn't typically result in comfortable conditions on both levels at the same time. That's what zoning is for. We don't know whether the replacement contractor proposed a zoning option. Absent zoning, balancing dampers will help compensate for differences in the heating and cooling demands of both levels. They may need to be adjusted seasonally or even during temperature extremes in a single season.

I had a package heat pump that featured a blower fan whose speed could be adjusted by moving a jumper to one of five terminals on the fan. The installer selected the second-lowest-speed terminal and air flow was poor. I selected a faster speed and air flow became satisfactory. Perhaps your unit's blower fan has such a feature.

-- amateur