Rubles without any problems can be exchanged in Crimea in any exchanger. Moreover, knowledgeable people do not advise changing money in the capital, much less risking the exchange of money in trains that are in the midst of the tourist season. This is explained by the ban on import of hryvnia to the territory of Ukraine by non-residents. If you decide to change currencies on the way, be cautious and make an exchange already in the territory of Ukraine after customs control, otherwise they can be confiscated at the inspection.

If you go with a card, the question is removed by itself: or habitually pay it off. It will be necessary to lose some commission, but in any case, the interbank rate is closer to the official exchange rate than in the exchangers of the resort towns in the high season.

Pay only the hryvnia, and change is definitely better in Ukraine.