Retired plbg1
It sounds like you have a 1″ main around the basement and you want to change it to 3/4 and go up through floor to each rad. and drop back down, well it dont work that way, you have a 1 pipe system that makes a loop and returns to boiler. Why dont you cut new pipes to Rad.and make them shorter are make a soufit around pipes that is what I did in my basement. Dont mess with heating unless you know what you are doing. 
