Well, it depends upon the context...
For example, if the 'rules' relate to the Government's tax legislation for small businesses, then once they change, the company must comply with the new instructions from the published implementation date. This may be good or bad for the business in question - eg depending upon whether their tax liability is going up or down as a result. Either way, there is a clear break point, after which the new instructions apply.
Something similar happens in sport. Between footballing seasons the authorities may implement a change to the way the offside rule works. From then on, that rule holds. Whether anyone understands it is not the point.
However, not everything is that simple. For example, every so often our building regulations change. When this happens, there may be a lot of existing structures that don't conform to the new rules. Depending on the details of the change in question, there may need to be a long process to bring existing buildings up to scratch (think asbestos removal, or more recently the removal of dangerous cladding after the Grenfell Tower disaster).
And as we've come to understand more about climate change, the necessary move away from fossil fuels has become more pressing. But plotting the right route through that change seems sometimes hard.
But what about D&D? In the last year, as I write, we have had a change of rules from 5E to 5.5E (or 2014 to 2024, depending on your taste). These systems are broadly compatible. So simple adventures, or modules can usually be converted. Illusionists and other subclasses work differently, but could be adapted. Other things need to be looked at, of course, from spells to the amended combat system, but this should all be straightforward, shouldn't it?
Well, perhaps not. There are a number of subclasses missing in the 5.5E ruleset as I write; for example there are no Necromancers, nor Echo Knights. If an adventure relies on these, then more wholesale restructuring might be required if the game is to be run under the new rules in the near future (later supplements will doubtless address these gaps). Or the referee could tweak a 'homebrew' variant of those missing elements, allowing the 2014 material to run fairly in a 2024 environment.
In some ways these are still the easy problems. Player characters also need to be considered. There may be considerable investment of time, thought and care into the development of a character's skills and personality. Wholesale adoption of the 5.5E rules could well remove some of the key features that the player enjoys about their character, or unhelpfully weaken some of their signature moves. The 5.5E changes to the Illusionist subclass, or the new rules for the Sleep spell might be cases in point. Again, a sensitive referee could find fair compromises with the player, perhaps exceptionally allowing some of the older features to remain within the new environment.
Often, those organising and playing in a long campaign may simply ignore external rule changes, aiming for consistency and stability within that game. Eventually, however, there may be elements of the new rules that they wish to incorporate. For example, some of the new rules for Artificers in 5.5E might be attractive, or a player might want to try the new Psi Warrior subclass. Occasionally, some part of the new rules may be retrofitted to the older game environment - essentially the reverse of the previous example; in this situation the 2024 rules are operated in a 2014 environment.
I suspect that quite a lot of games will make use of such hybrid rules for some time. And perhaps this should be celebrated. So long as the players are in agreement, and the adjustments can be made in a balanced way, that is fine.
The published rules aren't sacrosanct.
Which thought raises the question of 'homebrew' rules. From time-to-time a referee or a group of players may introduce additional or alternative local rules to fill a perceived gap in the published game, or to improve upon it. These might be small adjustments, or quite major. As I've written elsewhere, as long as this encourages good gameplay and narrative co-creation, it is to be encouraged.
But what happens when the published rules then change and fill that gap - perhaps in a different way?
Given some of the longer history of Orn, this was an issue encountered when the game came to be designed. Some of the rules I wanted to use in the game were developed to enhance version 1E, back in the late 1970s and early 1980s. In particular, an approach to magic specialisation developed by the late Ken Brown. Clearly, the Wizard class now has a number of well-established subclasses doing something similar, but I quite liked the flavour of Ken's older, homebrew idea. So they will be found in the Orn material as it develops, as an option. And a few non-player characters will be encountered who use those older notions.
So the rules used when the Orn campaign was first played were something of a patchwork, with options. But that's probably not to everyone's taste, and it isn't mandatory. The campaign can be run using the latest 5.5E rules, with only a few of the additions mentioned herein.
Or you could add some more ideas of your own, of course.