Although this is something that may rarely done well that doesn't mean that it can't be done or that it is always bad. It just means that it is difficult and that most people can't pull it off. It also may mean that some writers have tried it unsuccessfully. I wouldn't do it unless you have to but if you feel it's essential to the story then give it your best shot. In a first person POV there are always techniques to giving the reader whatever information you want them to have but they may not always let you give it to them at the most dramatic moment. I would much rather read a well handled shift of view point than the very common technique used by writers of first person POV that I call driving the plot by stupidity. In this the author , inorder to give the reader the information, allows the MC to see it but to always be too stupid to know what it means even though it's obvious to the reader.
Of course the usual way to handle this is, as mentioned before, to write the story in close 3rd person perspective so that the shifts are less jarring but there are sometimes that this just may not work.
If your primary concern is the jarring aspect of the perspective shift, there could be ways differentiate it as part of the chapter formatting, so that it is clearly differentiated from the main body of the text. The easiest example usually site is Ender's Game, where each chapter started with a bit of floating dialogue (no names or context, just the spoken conversation between two characters, and you are only later are able to guess at the identities. It seemed to be to be a clean way to present information/insight from the antagonists' POV, when the rest was tied to the MC's POV.
If its early enough in the story, Prologues can do this well, since the reader expects it to be somewhat separate from the main body of the text anyway. In one WIP I have the POV is tied close to the MC, but the prologue covers the famously unusual circumstances of his Birth, while I intend to use Chapter blurbs to show peaks into the POV of the people secretly hunting him during his pilgrimage.
