I don't think I replied to this thread. My car was mfg 10/06. It was Dec. 2011 and I had a "coupon" for a BMW OE battery replacement at $280, so I got it replaced. Dealer said why and I just said I want to do it proactively. They said OK but you don't need a new battery.
As I type it's August 2024. The battery from 12/11 is still installed. This isn't uncommon for at least the E9x. When I do replace the battery I'll let the dealer do it because my software "won't" register my battery. I guess it's because it's an early 2007. That's probably part of the ability to not overcharge it as it degrades. I'll be looking at the delta of lead acid vs. AGM, sometimes it's $100 or less. It's worth it to get the latter, because that requires coding in addition to programming. If I were to get the Costco battery, the Ah can't be exactly matched, but people say, "close enough."