I actually went to the Apple Store last spring with battery upgrade in mind, but Apple Store in Burlingame told me it will take 3-5 business days to do the change.

I didn't have a spare laptop to work with at that time for a week so i held of[1]. This is probably a reason for lot of people opting out of upgrades.

After the M4 came there was hype for the new screen which performs better in sunlight and M1->M4 seemed worth it plus vRAM which is certainly handy.

[1]The screen was also deteriorating - no physical issues just wasn't as bright as it was new) and they said I should replace that too. The economics a new Air worth of money on a older laptop with just 1-2 years primary use life left is shaky.

I just purchased a MacBook Air on the spot to bridge the week and returned it when I got my MBP back

Also both times I did it, it took 2-3 days, not the quoted 1+ weeks