I was talking about specific information of what changed, like this:
Not just a "New mission type: this". Most people, when seeing that, will think "Cool! I can see a new mission once I update" and not know that they actually cannot generate that mission since they'd need to find and write a whole bunch of stuff in their saves that they don't know how to find/write.
Also, there is no reason to remove perfectly fine code that can handle old save, so there is no reason why this would break saves. If there's "possibleMissions" in the save, read it the old way, if there's "completedMissions", read it the new way. It is an investment of dev time, of course, and that's why it's a suggestion, not a demand. They are free to determine how they want to spend that time and I don't think we have to worry about that.
BUT you need to cut this by the factor of at least 6 to see a mission, since most missions can be generated 5 or more times (once for each allowed region), so there's a good chance to see all missions in 3-year campain,
That doesn't help in seeing new missions, since they never got a possibility to get into the generation table. You have to wait 11 game years for Piratez to generate a reticulan mission if you upgrade now and don't tweak your save, because you have to wait until you ran out of all other missions for it to generate again. Then it should take only about 3 game years to see them
PLUS it is definitely my intention that some people won't see some missions during a single playthrough
Well, with the amount of content you already have, it's a good thing we don't need to see it all to finish
Waiting for the next playthrough can work too, but having the missions integrated in the current one makes it likely that it will be more novel compared to the previous, which is a plus.
Personally, I'd prefer a third option: a simple switch that tells the game to ignore 'available missions' and always choose at random (using mission/regionWeights) from the ruleset file.
An interesting alternative option, which would be easy to implement before "completedMission" since that would involve generating a whole list every load. "completedMissions" can then be built upon that, by removing what is listed in the save from the generated list.