We're drifting a tad off topic here. My reason for wanting to flip the culture of the province was so I could release Castille as a vassal giving her all her provinces except the CoT. Since that is not an option, I'll have to eat away at her (and portual, and Aragon), rob them of their CoTs and then force vassalage, force trade league membership and sell them their provinces back. What an infamy-heavy pain. I'll ease the burden by Holywarring the entire non-christian world since that seems to be the only bloody way to keep these heathans in my Trade League. Even with 200 relations and no CoT they are breaking away from me.
I haven't really had much of an issue with Portugal, Aragon or Castille, in my game. Obviously mods will be different, but at least in Vanilla the big colonizers rarely accumilate 500 cash (because of colonizing expenses I wager) or else they're always at war. Right now I've managed to nicely balance the three Iberian countries such that none are strong enough to wipe out the other. It's the landlocked big countries that are a pain - especially the ones that stay peaceful (ironically) because they can accumilate the cash to break out of the league.