Just an FYI for those unaware since this is a rare, potentially once in a lifetime showing. Its the first time the movies are airing in American theaters.
You can find ticket and theater locations here:
https://www.fathomevents.com/events/gurren-lagann-the-movie-the-light-in-the-sky-are-stars/
Aniplex's subtitles are soulless but its still Gurren Lagann. The second movie is also the actual good one. It covers the second half of the series.
Gurren Lagann is one of those rare and special series with a lot of heart, soul and meaning packed into it, still my favorite anime after almost 9 years when I first watched it as a college freshman. I consider it the greatest anime of all time.
I checked out the first movie on this Tuesday, it was okay, more tolerable in theaters than when I watched it on my PC like 1-2 years ago since the theaters audio and environment help obscure the flaws and issues with it. Its no substitute for the series though.
If you haven't watched Gurren Lagann before, you can just watch episodes 1-15, then watch the second movie if you want to catch the theatrical release. The series is still largely better but the second movie is good enough that I can still recommend it as an acceptable way to experience the second half of TTGL (just make sure you watch the series conclusion afterwards too).
So are the movies retelling later episodes with higher budgets?
I suppose that's one way to put it, yes lol.
There are some small (but important) changes in the second movie. It's almost like a "what-if" ending. It's also basically fanservice for Gurren Lagann fans in that it's even more over the top than the series in some respects, though you lose some of the depth and meaning of the original series since it cuts some things out.
The first movie is a lot worse, in that it changes too much and not for the better and leaves out some very important stuff in the process, but the second movie is a lot more careful and well-handled.