They don't tinker a ton in the regular season once they find something they like. They'll mix it up when they're in a funk, but I wouldn't classify Butch as a tinkerer. That said, they did spend quite a lot of time early this year auditioning players with McAvoy. Unfortunately, Lindholm's injury limited their ability to move him around down the stretch to see how he looks with other guys.
Cassidy said with the swap, they can have a one man breakout with each pair. That doesn't make a ton of sense to me, because they fliped two mobile guys. Really of the top 4, 3 of the guys could be one man breakouts. So nothing really changed on that front. My take is that with Grzelyck/McAvoy they are looking to maximize offense. In the past 2 regular seasons, when McAvoy and Grzelyck are on the ice together, the Bruins are outscoring opponents 50-17 in ~740 minutes of 5x5 play. It's an absurd differential, but one that is supported by expected goals. Furthermore, when McAvoy/Grzelyck are on the ice with Bergeron and Marchand the Bruins are up 11-4 in a much smaller sample, with a 76% xGF%. In Game 1 they played Lindholm/McAvoy with Hall/Haula/Pastrnak, while Grzelyck/Carlo was behind the Bergeron line. I think tonight they'll put McAvoy and Grz behind Bergeron to try and spark something for the top line. The Bergeron line was pretty flat in game 1. My thought is that the Bergeron line likes to go low to high and use the points more, so Grz and McAvoy are a better fit, while Hall and Pastrnak play more down low and thus don't use the points as frequently. Maybe the more active and mobile Grz/McAvoy fits better with Bergy, while the less mobile (and frankly really bad offensively) Carlo isn't such a detriment to the 2nd line who play down low more. Lindholm can trigger the breakout but once the puck is on Hall/Pasta's stick then the bulk of the work is done.