He wasn't in the middle of anything, the fight stopped and then restarted after Kuma's shock. And considering that the swords in the left would be by itself an ultimately impractical detail that only "affects" how the user faces a fight, then it makes total sense for him to switch the moment he's about to strike Kuma after a pause during which the entire Thriller Bark lost to him. What's the difference between taking one second to put the bandana on and taking one second to switch sides, for example? They'd both be impractical actions that only obey a psychological reason for Zoro (it's not like the bandana always equals being serious, but well).
Proportionally, facing Kuma in Thriller Bark was more dangerous for them than facing the Beasts Pirates now because, well, we know they are going to win now but they were dead meat agains the former. If Zoro feels threaten now, imagine back then.
Again, it is bullshit and, if not a drawing mistake like many others, just an aesthetic choice for Robin to be more visible. In almost three decades of serious fights Zoro has never, ever been consistent on putting the swords on the left side. It means nothing.