I did not read all the replies, so hopefully I did not miss important information.
I am putting myself in the shoes of your guests. Personally, I view this upgrade as a legal one and not one in your actual relationship. You are already committed and married in my book! If I was close to you, then I may be happy to celebrate with you. If I was a more distant friend or relative, I may feel like I am being asked to do a second wedding. In fact, I would likely decline the invitation feeling like I had already celebrated your commitment to each other. Either way, I would feel obligated to bring a second gift, which would honestly irritate me a bit. To be clear, I would be (and actually am) very happy for you.
I think the key is to make this look like less of a wedding reception and more of a party. I would likely keep the party smaller and to those who are close to me. If I really felt the need to have another big party, then I would take great care in how the invitations are worded so that it does not sound like a gift giving occasion. I would also not register anywhere. I would also put the word out with some trusted individuals that this gifts are not anticipated in the hopes that people will not feel obligated. But I think that this would be tricky, as I would likely feel like I should bring something. I would probably forgo some of the traditional wedding reception activities, especially if I did them the first time. I would truly try to make this more of a party.