I have printed out this thread and am hoping for warm weather (relatively) this weekend and decent conditions so I can make the change.
I'll be taking out the glove box, too, to see if the hose behind it is rotted out and if that makes a difference to the flow to the center vents in the dash.
I think I'll have to leave investigating the "peeling" plastic I can see through the center vents until the summer. Longer days, more light, warmer temps, fewer gumption traps.
Thanks everyone.