HOWEVER, it wasn't tested or even intended to work, and you will certainly loose some configuration settings (assuming you use those parts that were changed.) and end up with a 'dirty' configuration some tags could be there that shouldn't be, other might be missing that are normally always there..

When going to a newer config version 'upgrade steps' https://github.com/pfsense/pfsense/blob/master/src/etc/inc/upgrade_config.inc can involve modifying existing settings into a newer 'format' restoring the new config format on a old pfSense version will likely mean those converted settings are not understood by the software and effectively 'skipped' or cause other unexpected problems..

I'm not saying its impossible, and well sometimes like in this case the config didn't change and it can be done without a possibility for trouble.. But be very careful and double-check everything is still in working order when restoring a config on a old version.

Usually a increase in version number also means some config upgrade steps where added.. so your a) and b) are done together, and yes tags get deleted/replaced or little changes to their meaning..

My 2 cents :).

a) the version number is increased

<pfsense><version>17.3</version>

b) tags are added. (have tags ever been deleted?)

Generally it does not mean that you don't use XML anymore or do something else that I'd call a "format change". It's still xml version 1.0, right?

An older version pfSense would have "problems" reading tags that it doesn't know yet.
Would it stop importing all infos of a newer config.xml or just skip the unknown parts? (I don't have an install handy to test it with).

Isn't calling it a "change in configuration format" a bit too much?
But then, I'm not native english and I might be missing something.

PS: Really no pun intended, just trying to understand!</pfsense>

If anything gets changed on 2.4.3 snapshots that increases that number, then the configuration can no longer be moved back to 2.4.2.