Converting a return into a normal vent?

Red Squirrel

My house is a split level and I have a inwall return by the ground in the lower end of the house, and a bigger return in the higher end of the house as well. I find I don't get enough heat on the lower end. I'm debating on converting the return into a single air vent, or simply extending the duct in the crawlspace and feed a few vents off of it so they get better pressure. I think the main ducts they used are slightly too small so when all the vents in the lower end are open, the air is spread out too thin. That's my guess anyway.

The return I remove, I would replace it by adding another return in the basement, where the server room will eventually go (so I can suck out the hot air + warm up the house with it along with the furnace). Think I would be safe to do this? Basically the number of return volume would not change, but I would add a whole large duct worth of new venting.

I've never worked with ducting before, what kind of tools are needed for this? I'm guessing I need some kind of cutting tool, and a rivit gun.