I dunno. I don't even know much about polling... I suppose if a series of polls that are statistically robust & directly comparable in all regards had been showing a steady move away from pro-brexit to pro-remain then that must mean something important. But I'm not sure if that's the case here?
I could easily see why pro remain sentiment might be increasing. At the time of the referendum I suppose I thought that Brexit would mean we'd become something like Norway, relatively little change, I suppose 'liberate'' ourselves from the shackles of a few relatively unimportant banana-straightness type rules, get a new PM [probably Boris], stay in the single market [as was, to the very limited extent that even the most important details such as this were being discussed at the time, promised], keep paying our subs [albeit for a smaller say], almost certainly keep freedom of movement.
This idea that there can be a real possibility of cutting ourselves off entirely, just voluntarily damaging the economy so badly, was to me at least new & shocking and would, if widely known at the time, surely have knocked at least something off the support that Brexit had at the time.