One Down Kit = Not using a Reno or One Up Kit when you advance an Age.
If this were an actual proposal I'd vote no.
Well, that's obviously not true, and it's so obviously not true I'm sure you realize it.
Sometimes you win something in an age, and you have not upgraded it. So, no reno kit, or one up kit. You just won it that age.
I would vote yes for that, under the condition they were very rare, much more so than reno kits or one-up kits. If they were common, it would be a mess. If you got three per year, I'd be OK with it, because it's only one age and you'd probably want to move stuff more than one. If it were pick an age, it would have t be one per year.
If either were like reno kits are now, total disaster. Knowing Inno, it could easily go that way if they could get diamonds for it.
But, the one fundamental flaw that seems pervasive here is assume this is for GvG, which is a dying part of the game. It certainly could be used for that, but it could also be used for other purposes less destructive to other players. For example, the main thing I can think of is down-aging Terrace Farms or something similar to lower the population requirements, assuming the person were using it for forge points. If done in extreme moderation, they could improve the game. If done excessively, it would cause far more problems than it would solve.