This question is strongly related to this one: Balance between character's point of views
However, that question is asking about balancing POVs in general, whereas this one is about potentially subverting reader expectations by introducing a new viewpoint late into the story.
I'm currently plotting a Romance story. Given the events I'm planning to take place I can already see which character perspective would be best suited for each chapter. It looks like for purposes of character development, the majority of the story (basically, the first 75%) will be told through character A's perspective. However, in the last quarter of the story an event takes place that's only experienced by character B, so there's no way (other than completely changing the entire premise) to get around adding their POV near the end. (I guess I could have character B tell character A about it, but that doesn't sound particularly appealing, either.) This is not a problem in itself. In fact, I rather like that this gives me an opportunity to show the romance from the other perspective.
However, since the first 75% of the story are told from one character's POV, I wonder if the sudden introduction of a different viewpoint would be jarring to the reader.
FWIW, the story is going to be told entirely in 3rd person limited. Both characters appear in all chapters, so this is not about introducing a new character, simply about switching to their viewpoint.
I had been planning to have each chapter dedicated entirely to follow one POV, but now I'm wondering whether it might be worthwhile to add the occasional POV switch early on (on scene changes within a single chapter) to get the reader used to this taking place. I've already identified a few points in the story that would lend themselves to a POV switch, but from a story perspective it's not strictly necessary and I'd prefer to avoid doing so unless there's a strong reason to do so.
And of course, all of this might change completely once I actually start writing this story.