I have a 1950's ranch with galvanized supply and return lines that connect to copper baseboard radiators. It appears they are using a brass or copper fitting that is soldered to the copper below flore level. There are plugs on the bottom of all these fittings. The strange part is that the galvanized line appears to be soldered also, although I assume it isn't. Some of these pipes are bent around corners, really quite nice except I have heard they rot from the inside out. I would eventuially like to replace these, one zone at a time, in case I screw up, I will still have heat. Any thoughts on copper, cpvc, or pex to connect new slantfin radiators, a few at a time. Is it possible to lock out one zone a time. Also if I turn my water off for plumbing repairs will this effect the heat or are they closed loop systems. Thanks all help and advice appreciated

((thoughts on copper, cpvc, or pex to connect new slantfin radiators, a few at a time))

They will all work well but CPVC/PEX is easier to work with.

((is it possible to lock out one zone a time. Also if I turn my water off for plumbing repairs will this effect the heat or are they closed loop systems. Thanks all help and advice appreciated))

No one here would have any idea because it all depends upon how the pipes were laid out originally and I just cannot see that far.
((also if I turn my water off for plumbing repairs will this effect the heat or are they closed loop systems. Thanks all help and advice appreciated))

When you break a system open after shotting the water valve off you will have no heat until the repairs are made. Now you might be able to cap off a section but that depends upon how the pipes were run originally. See note above..

There are two zone, one is a normal zone that was added for an addition, its all copper and newer. The second is I guess what the call a dual loop? One zone branching out for circulator into two loops, each with a shutoff a couple of feet from the circulator. They each gop sown opposite sides of the house with 3/4 galvanized and the at the other end of the house join in the middle, at which point the return is 1 inch galvanized, and back to the boiler. I would swear that these galvanized pipes are soldered, but I wasn't aware you could do that. This summer I want to replace them, was thinking pex, and thought I'd add the kitchen and living room to the newer line as it only has one room on it, and then just making one loop for the 3 bedrooms and a bath. Any thoughts

Go for it. Sounds like you know what you want to do.