Civil unions were introduced in the Netherlands in 1998 and same-sex marriage in 2001. A registered partnership could easily be converted into marriage (and the other way around, but this is not possible anymore since 2009) for a small fee, but it didn't happen automatically and I don't think people received a letter. Civil unions still exist, for straight people too, even though they are pretty useless now.
Interesting ...
I think it's pretty stupid that you probably have to pay a fee here for converting the civil union of a couple to marriage status, just because the state was not willing to introduce gay marriage in the first place.