I would assume it would be similar to going from D5->D7. At least one HUGE component has changed (DBTNG), plus Fields are now in core....plus a whole bunch of other stuff.
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Cameron Eagans
Owner, Black Storms Studios, LLC
http://www.blackstormsstudios.com
I'd be interested in hearing opinions, on a % scale, of how much reeducation is needed to move from D6 to D7 in terms of the api, hooks, architecture, admin functions... for a D6 developer to be at the same level of capability with D7.