A civil-partnership-converted-into-equal-marriage is something that is very much worthy of celebration, and I'm sure everyone who attended your wedding will be very happy for how this change in the law affects you and your wife.
So personally, if I knew someone in this situation and was invited to a big fun party to celebrate, I'd absolutely go. I'd definitely give them at least a card to mark the occasion, and possibly some sort of token gift as well, but only because of my own feelings of happiness and not out of any sense of obligation.
In other words, I think what you're planning on doing is fine, although perhaps without the wedding dress, if it doesn't match the formality of your party. Basically, if you wore a knee-length cocktail style dress when you got married, that would be fine for a moderately fancy party, but I wouldn't wear a full-length ballgown style unless you expect your guests to be wearing that level of formality, too.