Moving cold air duct

[Andy T. Bishop]

I need some assistance.
I want to move my cold air duct to make room for a large toy box.
Right now it is in a wall and follows about a foot along the floor joist and they is a hole pop'd in the return. I want to move it to the end of the hall, 4 feet away and put it flush into the floor and tie in.
Do i have to close off the old hole?
Do i need to run duct work for the new one or can i just POP a hole in it and have the grate above it? I think i will run a piece of duct work but thought i would ask.
Thanks
Andy B