If this is a problem in your games, it strikes me that there is a very simple solution. Just record exactly the state of your warband in between each game. Then you will have a record of it for every game level you've played, and will be able to recreate it.
If you are playing someone who has only played two game (and you've played loads) simply field your two game warband, instead.
This solution also allows you to nail on whatever handicaps you wish. Eg, you could say that you couldn't have a disparity of more than 3 (coz you want to reward more powerful warbands, but not too much). So, against a two game warband, you could field a fiver.
Any exp and treasure you win just goes on to your running total.
If you need to construct a linear narrative for your band, then this might be a bit of a problem, coz you will effectively have parallel realities, or something. But in a game that includes Chronomancers, istm, this is a reasonable amount of timey-wimey.