PD always changes at level 20, not any other level. So if Man gets longbowmen, as seems likely, they come in at level 20.
I am pretty sure that Man's PD under 20 is just slingers and heavy infantry. I don't think it's that great. One of us could just check our manual though.
(Mine is up a whole flight of stairs!)
In any case Xietor is right that the mods would make no difference to PD, and anything else affected it is also very unlikely.