[quote=@Windicator]
what would reverting to fresh mean in terms of game 
[/quote]
An good example would probably be the first time Agumon Digivolved to MetalGreymon, when the period of being at the Ultimate level ended, he reverted back to Koromon.
Similarly speaking, the first time Paildramon was formed, if we were to 'assume' he was a Mega for the moment, when the period of being at the Mega level ended, they would revert back to Chibomon and Leafmon.
This is just a balancing rule so that we dont have anyone abusing the Evolution system.
As for run time of stages.. no. Rookie is the most basic of stages for a Digimon to maintain, it may be true that as time goes by it gets easier to maintain forms, but at no point should the Digimon simply stay at the Champion stage. Tailmon was a special occasion in Adventure, but i believe the reason for her Champion maintaining was the fact that she had WAY WAY more endurance for that sort of thing. If you wish to have a Champion level digimon as a partner, however, that will have to be discussed, as certain digimon work and others dont.

EDIT [@sMoKe] on a side note i totally found a cool evolution line for Hackmon i thought i would share? Imgur is blocked for me atm so hopefully this works http://i.imgur.com/jdWJWio.jpg