The primary piece of Bellichick's coaching that I admire (and it pains me to admire him) is his willingness to IMMEDIATELY make in-game adjustments. Sometimes it seems like the first couple drives are diagnostic in nature, and once he and his staff accurately assess the opponent's plan, they set about countering that plan.
Obviously that's not always necessary, or at least major adjustments aren't always called for (based on well-scouted gameplans and/or superior players/execution), but I've seen countless clips of him meeting with coaches and position groups on the sideline, detailing changes in approach and with specific techniques.
Halftime is a great opportunity to lay out more comprehensive plans for the final 30 minutes, but I don't think waiting to tweak or adjust or even scrap certain gameplans makes any sense.