There are plenty of rational reasons that someone might have come down on the Remain side before but swung the other way now. The EU has arguably proved several of the major criticisms made by the more reasonable Leave supporters correct since then. There was criticism of the democratic deficit. Now we have Ursula von der Leyen running the show and (figuratively speaking) no-one is quite sure how she got there. There was criticism that the EU was excessively bureaucratic, litigious and slow-moving, and that member states were fair weather friends. Then we saw the handling of the COVID/vaccine and immigration/refugee issues. And the more extreme predictions of British life as we know it coming to an end haven't happened either (at least not because of Brexit).
Of course, there are also plenty of rational reasons that someone might have come down on the Leave side before but now think it was the wrong choice. Several of the major warnings given by the more reasonable Remain supporters have also proved to be well founded, particularly in terms of the immediate consequences of the extra trade barriers and the limitations on free movement of labour.
I'm not taking any side here, just pointing out that both sides had legitimate points to make before Brexit that have subsequently proved to be largely correct, and that someone who didn't expect that might have changed their view in light of that information, in either direction.
It's perhaps worth remembering that the older generation who voted more in favour of leaving in the Brexit referendum were much the same generation whose younger selves voted to stay in once before. Obviously the nature of what is now the EU had changed a lot in the meantime, and so had the balance of public sentiment within that demographic. It might be surprising if such a large shift had occurred in just a few years now but the basic premise that some people might have changed their minds since voting on Brexit is plausible enough.