Coolant questions

[Mayszak, David A]

I need to replace the radiator fan thermo switch and have noticed that the
previuos owner installed "green" coolant in the system.  In replacing the
switch I'll probably loose some coolant and have replace it.  The car has
143K and nothing tells me that it isn't the original radiator.  My question
is,...at this point, should I replace all the coolant with the orange
(phosphate-free) stuff???  or just top off with the green stuff and hope for
the best.