DIY? Replace low hanging boiler lines with pex?

Awesome site. Appreciate ideas, comments on my issue. We are in a 100 year old modest character home, 625sq ft per floor, 2 floors and an unfinished basement, with a 25-30 year old Burnham water boiler with newer circulation pump, attached to large dia(2.5" reducing to 1" at the end of the run) iron(?) supply/return lines that may be from the 1930's. These supply cast iron rads (approx 2'x3'each), one per room. Basement is heated only by the exposed heating lines. It all works fine at this point, reasonable heating bills, no concerns. Time to finish the basement, so those low hanging (approx 5' 10" off the floor) heating pipes have to be raised to the joists. A neighbour told me about his friend having it done with pex. I am handy and would take this on if it can be a diy job. We would be happy to simply replace the basement lines at this point, and add some basement rads, and do nothing else. Can we simply run basement pex and connect to the iron lines that come from the rads thru the main floor into the basement (from both levels)? I don't want to get into fishing pex thru walls/floors to get to the upper level at this point if i don't have to. Is it necessary (i imagine advisable) to ensure the supply/return pex is always at or below the level of old rad pipe connection point in the basement, to avoid air locks, or is this really a concern? Some of these pipe fittings occur (not