If I declare UMH with my partner, is it possible to get married afterwards?

Yes, as long as it is with the same partner you can get married without any inconvenience. Bear in mind that if you want the assets acquired before the marriage, during the de facto marital union - UMH, to form part of the marital partnership, you can previously declare, by public deed, that you have had a de facto marital and property partnership between you and that you want the assets forming part of the property partnership to enter the marital partnership that arises with the marriage. This deed is called "declaration of assets of the undeclared or unliquidated de facto marital property partnership that enter into the marital partnership". In this deed an inventory of the existing assets must be drawn up.