My inexpert views: If you’re not already at a level of acquaintance suitable for discussing personal life stuff, don’t bring it up at all unless she does. And unless the previous occasion of your meeting was particularly noteworthy, “Hi, I’m - we met once a few years ago” should cover past history.
Key points are to make sure to use the new name and appropriate pronouns, and avoid staring or overtly gossiping with others about her. Basically, common courtesy rules apply as usual.
One possible thing that might be cool to bring up, if and when the conversation goes in a direction where it makes sense to do so, is if you ever recall having thought “that guy is kind of girlish in some way” back in the day, mention it.
I specifically met this person (before she came out) because she has a transgendered child, and so do I. So meeting again will be strange because this is the very topic we discussed on our original meeting. I probably should have mentioned that in the OP.
I concur with the “Hi, we met a few years ago. You look well.” Otherwise, stay away from comments about appearances unless they are very positive, i.e. “that haircut is very flattering” or the like, probably best to say nothing in that regard at all.
Otherwise, stick to normal subjects for re-meeting an acquaintance. The more you can treat her like just another ordinary person the better.
Of course, there are other ways it can be awkward, as well. Like, at a cousin’s wedding a couple of years ago, my aunt’s ex introduced herself as the “father of the groom”. Which was certainly biologically true, and was how the groom was raised… but I’m not sure if e identified to emself as female or male at the time. And when referring to events in that past time, what pronoun is appropriate?
Following this one because I just found out a few weeks ago that my cousin’s daughter is in the process of becoming her son. My dad broke the news to me as we were driving to my aunt’s birthday party because we would be seeing all parties involved, including the transitioning person’s girlfriend. I am at a loss for how to deal with the whole thing and don’t see any of these people often because they all live in other time zones.
It confirms a person’s identity. I’m not surprised that it sounds to someone like a very bad idea, and it may in fact be a bad idea, but if it were me I would feel validated by that. OTOH, I don’t identify as transgender so maybe I don’t know WTF I’m talking about, projecting onto them that way, as if what I would appreciate would be appreciated by them.