Its a lot easier to dictate match ups rather than adjust to other teams when you have:
1. MVP caliber player in his prime (KG)
2. An all-nba SF who is skilled anywhere on the floor
3. An all-star SG who still could have scored 26 PPG as your third option
4. The best defense in the NBA
When Doc hasn't had that, he's had to be more reactive. Which is the sign of a good coach, you can't just "stick with your identity" if its not working. Now the C's never got crazy out of character or out of system, but they adjusted quite a lot as veteran team.
That OKC game last year comes to mind, right before the hot streak. The C's were getting blown out and the players went into half time talking adjustments. IIRC Doc cursed them out, said they weren't changing anything, and that they needed to play harder. Every coach prefers to use "their" system, but you can't just go down with a sinking ship if your personal or talent doesn't support it.