I've been down on the Celtics chances, because I fundamentally don't believe in "flipping the switch", particularly if you don't have a core that has won a title together. The regular season is important, and bad habits picked up in the regular season can carry over into the playoffs.
While I can't think of a non-championship nucleus that successfully "flipped the switch", there's an obvious example in the fairly-recent past for the Celtics of a core that did so: the 2010 Celtics.
I've tried to block the 2010 Finals out of my head, because it legitimately still causes pain. 13 years later, it hurts. There are lots of reasons why it hurts: losing to the Lakers, Perk getting injured, and mostly the 4th quarter screw job by the officials. But, for those looking for hope this season, let's not forget how the 2010 run happened.
We started the season 23-5. We finished 52-30. In other words, after the hot start, we went 29-25. We limped into the playoffs losing 7 of 10 games. We ended up the 4th seed.
Hope was pretty much lost. Then, we beat the #5 Heat (Wade), the #1 Cavaliers (Lebron) and the #2 Magic (Howard), with no series going further than 6 games. It was a run that absolutely nobody expected, and we took a 3-2 lead on the Kobe Lakers, the best team in the West.
So, that's the example we need to hope for this season. Excellent teams -- championship-level teams -- can mess around during the regular season, and still have playoff success. I wouldn't be on it, but there's precedent.