Hello,
folks probably will hate me by remembering that, but I did a rather smooth update some time ago in a Raspberry Pi (1) with ext2/3/4 filesystem.
By upgrade understand going from 19.0.x to 19.0.x+1 and not from 19 to 21 or similar.
Here you may find a draft recipe: Upgrade from 19.07.0 to 19.07.1: does it require sysupgrade? - #4 by lramos.
Problem is that OpenWrt is designed having commercial routers in mind (a system with R/O firmware+RAM), and not for Linux PCs plenty of rewritable, nonvolatile space. So it's more natural to make sysupgrades from time to time rather than doing apt-something or like daily.
Then I wonder about the existence of opkg upgrade. It's a marvelous tool, but it doesn't fit very well in this world.
I suppose your case is about routers (that is, you're trying to do incremental firmware updates). If so, please do not try my recipe; it much likely won't work. However if you have a system with R/W nonvolatile memory and use ext2/3/4 it may work.
Anyway, like before, I would welcome a good solution for a way to do incremental updates. Mainly because for the security point of view, responses to threats should ideally be deployed as sooner as possible.
Regards,
Luiz Ramos
Sao Paulo - Brazil