Walls and Ceilings - taking a wall down between Kitch and Fam room - ductwork in way?

We want to make the entry from our Fam Room to Kitchen bigger. the problem is a large duct serving the second floor. It is 8" by 20" and runs right up next to the door opening.

I would like to re route this ductwork and widen the opening.

1. What calculations can I use to make sure I maintain air flow for the second floor?

Can I convert the 8x20 chute to two 8-inch round flexible pipes?

2. what other options are available? could I build a box in the wall and line it with material rather than using standard ductwork?

I have heard about a 'chase' what is this option and does it impact the shape of my wall?

thanks for any help!
best regards,
Eric

I would wait for an expert in HVAC to show up. It seems that maintaining the same cross-sectional area of the ducting would allow the same supply. Since two 8 inch round ducts would have as much area as a duct 8x20, it looks ok. I imagine that there is a limit to substituting one size for another, because of air turbulence in smaller ducts. For instance, it seems that substituting small diameters such as 4 inches might not transfer well.

I know that some duct work, such as return air is sometimes done with the flooring and joists and sealed well, but return air might be different to work with because the pressure issues may be different pulling the air back instead of suppying it to the house.

Rigid duct work comes in different sizes, and it may be that you could take some and make your own duct work from it. This may present some challenges when it comes to connecting to the rest of the system.