there's three bits to making sure replacement names work;

each name in descr_names.txt has to have a corresponding entry in
data/text/names.txt - if you're only using existing names that shouldn't be a problem as they must already be in there.

each name used for a faction in descr_strat must be in that factions section of descr_names.txt
(and in the correct gender / name / surname subsection)

If you're replacing the names in a family tree make sure you swap each instance of the characters name with the same name.... the same character will for example have an entry where he's listed with his army, then he might have two entries in the relative section below - one where he's the son of somebody, and one where he starts his own line as the dad / husband of other characters... make sure you've replaced each with same thing or you'll mangle the family tree