I seriously doubt that Pope Francis is attending with the purpose of celebrating the Reformation. Instead…imagine this for a moment.
Suppose you’re a Lutheran, and you attend this particular Lutheran church. On normal Sundays that are close to October 31, you might actually celebrate the Reformation, insofar as giving some recognition to certain Reformers, although there might also be a touch of regret for the lack of unity. But if you’re a Lutheran, you’re probably putting a good amount of blame on the Catholic Church, and this is a Sunday on which you will look to the positive things on your own side and celebrate them to some extent.
Now imagine that you show up at church for an extra-momentous 500 year anniversary. It’s still something that you have mixed feelings about, but 500 is a big anniversary. The Catholics never wanted the Lutherans to make it this far, but look at us now, 500 years on! Check that out! Take that, Catholic Church! And then the Pope is here. And you’re celebrating the anniversary (to whatever extent you celebrate it) with him present.
Ok, so this is a celebration that’s going to be a lot different than usual. As for its larger effect- I assume all Lutherans will be made aware of this by the time that date comes around, and Lutherans everywhere will look at the 500 year mark as a watershed moment, where those first 500 years wind up being significantly different from the next 500 years. This is something that Catholics in general always hoped would be the case anyway, and they hope that something can swing back their way, but when you watch from a distance and hope for something without influencing it in any way- good luck.
I look at this as the Pope making his own luck to the benefit of the Catholic Church’s interests. He’s getting right up in there- in a perfectly nice and friendly manner- and he’s lending his influence to the course that the Lutheran Church will take in the coming years/decades/centuries. He’s not a key decision maker when it comes to the fate of the Lutheran Church, and the papacy hasn’t had that role in quite a long time. It might not be too much of an exaggeration to say that the Catholic Church spent a whole century on each of the “five stages of grief” that finally leads to acceptance of that fact- but on the whole, if you do accept that the leader of the Catholic Church does not have any decision making authority among the Lutherans, surely you would prefer to see him exert some type of influence instead of settling in the direction of having none, or virtually none.