Quote Originally Posted by Aenarion
I would like to answer to this question, but I'm still a beginner in modding, so I don't wish to guide u to the wrong direction!
I had to have a quiet chuckle to myself at your skilfully worked non-answer! Reminded me that one needs to be circumspect about dishing out advice or suggestions about modding. Anything one is able to think up, as a 'plausible' answer but hasn't actually tested out for oneself, could badly screw things up for someone else.

Still, it's good to know that there are others out there actually reading this forum, looking for the same answers.

As it happens, I've answered this question for myself. I went ahead with some changes, returned to the campaign gamesave and it loaded okay, with no apparent data corruption.

As for the changes themselves, one data item initially prevented the game loading and I had to make further, incremental changes, as and when internal errors became apparent.

I'm about 90% done on successfully adding a new agent unit but it's not particularly crucial to gameplay and done merely as an exercise in finding out what is required to make it work. I'll start a thread about it, shortly. I'm convinced there used to be a thread about agent creation but when I seached, very few references came up. (I think recent forum servicing caused search to be temporarily disabled though).

I still think that certain fundamental changes will cause severe problems in an ongoing test campaign though. For instance, I changed stats for ships, adding one movement point to all the types (so the long-distance models retained their advantage). I now have second thoughts about this change (coastal range ships given movement allowance of 2 gained access to deep sea areas, before compass discovery, for some reason) but worry that unit properties get stored in gamesaves and a change back to default may trigger erorrs.

So, for now, a qualified, "yes, modding on the fly is possible".