Some good discussion here. Where I stand at the moment:

DC's for tweeners - I still like the idea. Let the market decide. Biggest issue is to codify a 'tweener' and spell it out in the ITCS that those types of 'border cars' will be the only ones considered. Hard to do and a hard message to tell someone their ca is NOT a tweener.

DC's for cars that move up or down a class - 1 year DC. Specific mention on the spec line with expiration date.

DC's for everyone - do it locally.
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I think Andy's conclusions are good here, though he didn't bother to support the last two.

Specifically, i think the last point needs expounding upon. I see this as a "track time" issue. In general, there are (many) regions where track time is precious commodity, and others where they are struggling to get people to fill the grid. Allowing the DC at the former could cause some people to not be able to race while others gorge on the track time afforded the DC. Whereas, it could be a great boon for the latter struggling tracks, allowing them to get the revenue associated w/ filling out their grids.