As far as I recall, it isn't an oversight that the Outremer melting pot doesn't change the province culture - as Zeress says, it's to represent the fact that the rulership changed "culture" in that situation but the locals did not. This does not prevent the province culture from changing in the usual way as long as the province in question is its owner's capital or a neighbouring province and its owner have already adopted Outremer (or whichever) culture. (however, cultures change much more slowly if there is no neighbouring province and province-owner with the "target" culture).