Couple things in response/clarification:

-- Yes I want to keep the radiators, like them a lot.
-- I am not too concerned about basement waste heat, currently have a
teenager living down there. (If he's good, someday we might let him
come upstairs.)
-- currently some of the (steel) pipes to the upstairs radiators go
through outside walls. While that has not caused problems in the 10
years we have occupied the house, it certainly is not real
energy-efficient; one purpose of redoing the pipes would be to get away
from that.
-- mainly I am curious if anyone has done this sort of thing and has
any tips. Also, curious if it would be practical to use CPVC or other
plastic pipe rather than copper.

-- H