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PU Russia (or any Big Blob like Austria, France, Spain, ...) and you can expect that they will convert quite a few land provinces for you. Old primary culture may stay accepted but other provinces from their old culture group (and of course anything outside their culture group) will get converted as soon as they are not accepted (breton and other small cultures in France and Iberia, tatar, finnish, byelorussian and ruthenian etc. ..., I dunno how cultural unions will react - e.g. if I change French primary culture to some other - I expect that it will even more bugged than it is now).
My concern is that it might be able to get PU on Big Blob as a medium country (using other allied Blobs to beat them), and if they are below 50% LD you force culture change. Now they are 99% LD, get support and you grant them independence. However after say 100 years all Siberia, most of East Europe etc. is not Russian, but e.g. Lombard/Bavarian/... , or big part of Iberia is Danish.
So my plea to Paradox:
1) Please add mechanism to reject/revert culture change, e.g. after declaring independence war
2) add more constraints in addition to 50% LD (say total tax (development), same religion etc)
What makes me sad is that Paradox decided to invest into broken culture mechanism (and I do not think that is anybody here who will say that current culture mechanism is fine: accepted culture magically gained after 3 years of coring process and later lost after 200 years of acceptance, broken cultural unions, possibility to erase/overwrite culture in a few years (e.g. Austrian Lowlands and Hungary, English Ireland, Swedish Finland, Spanish Italy etc.) , ... just to name a few), instead to rework it.