The Linux Mint upgrade instructions begin by saying:
If your version of Linux Mint is still supported, and you are happy with your current system, then you don't need to upgrade.
The situation I sometimes find myself in is that I am happy with my current system, but my version is not supported. In particular, I am leery of doing a giant upgrade that upgrades everything and may disrupt things. But if I don't upgrade, when my version stops being supported, I can no longer install or upgrade individual packages with apt-get.
Upgrade instructions for Ubuntu-based distros (such as these older upgrade instructions for Mint) often say that basically what you do is point APT at the new distro, and then upgrade your packages using upgrade or dist-upgrade.
What I'm wondering is, is it possible to "update" my system in the sense of making new packages (or new versions of existing packages) available, but without actually upgrading anything, and in particular without upgrading everything all at once? What I would like to do is to retain the ability to install individual new packages, and new versions of already-installed packages, but without ever doing an overall upgrade of everything on the system.
This may seem like a weird goal, but I often use Linux on computers that I use infrequently. It is very frustrating to fire up an old computer to do something, and find that I can't install anything because the apt sources are gone, and then have to cross my fingers and try an upgrade that may break everything. I would prefer to take an incremental approach in which the need to install or upgrade a single program doesn't require taking a leap of faith and upgrading the entire system.
I'm also asking this question because, just theoretically, I'm curious what the upgrade process actually does. If everything on the system is defined by packages, what is the difference between "upgrading to CoolPackage version X on Mint 17" and "upgrading to CoolPackage version X on Mint 18"? In what way does the OS version itself actually affect the upgrading of packages? Or, most extremely, if upgrading the OS is just upgrading all the packages, why do you ever need to upgrade the OS per se at all, instead of just upgrading each package (and its dependencies) as needed? I'm also curious if different distros make such incremental upgrading harder or easier.
(Note that I'm not talking about ignoring dependencies; I realize that upgrading a package may require upgrading specific other packages. But I wonder why there needs to be the notion of an OS-level upgrade rather than just each individual package's notion of what needs to be done to upgrade that package. Is the answer just that, after enough time, all packages wind up indirectly depending on new versions of fundamental packages, so that upgrading any package would effectively require upgrading everything to keep dependencies satisfied?)