you want the dynastic name? like the surname for after they're no longer princes? hmmm

good point, can't say i've ever really thought about it

ok
go to loc/eng/names.txt
look for the surnames for the faction your editing.
WAIT
first, find out the currant surname from the game
do the above
the royal surname is most likely to be the first on the list, but it may not be (hense the above)
when you find
[Plantagenet] {Plantagenet} (English is my example here)

then change it to
[Plantagenet] {Tudor}

the forenames is easier
in the forenames list, the first 8 names are used by the kings (except for french, where its 5)
edit it the same way

now go to campmap/names/default_heroes.txt

make the changes in exactly the same places in each names list
this file doesn't have the data string [] only the {} part displayed normally, which is why you have to edit it in the same order.