Changing the point of view of your story is typically frowned upon. There is one type of story, however, we're changing your point of view--or at least the expression of your point of view-- is necessary for the story. Though this point of view doesn't necessarily have an official designation, I like to call it the reminiscent point of view. I call it that because the story is framed as a reminiscence. In most cases, the reminiscence starts with the character firmly in adulthood until something happens to trigger a past memory. Because everyone changes as they get older, the point of view immediately changes to match the old self of the character. In some cases the difference will be negligible, but if you're dealing with an adult reliving a childhood memory the point of view and entire writing style will, of necessity, undergo a metamorphosis.
With this type of story, you might opt to use the same point of view for both present and past, altering only the voice of the character. This might mean using a third person narrative for both parts of the story. Or perhaps you'd like to use first person. Either can work effectively.
But there's a third option, one which makes a reminiscence stand apart from most other narratives. You have the option to work in both first person and third person for the same character in the same body of work. The present might be written as third person well the past is written as first person. And you could also opt to reverse this. Most other types of stories don't allow quite so much versatility.
But a reminiscence requires more than simply shifting your point of view. There should also be a shift in the writing as thoughts and actions are betrayed by the past self. Syntax, vocabulary, idioms, and styles of speech may all be different. Sometimes dramatically so if your past and present characters are separated by a great many years. A grandparent doesn't speak or think the way a child might, after all. This can be a difficult thing to accomplish and usually takes much practice. Playing around with the reminiscent point of view can enhance your writing skills and further your understanding of the nuances of writing a novel. This is especially true if you explore the many variations a reminiscence allows.
Personally, I don't often write a reminiscence, though I will sometimes insert a small scene detailing a memory a character is had. But since these tend to be recent memories, they aren't quite a reminiscence. If you like to play around with different styles of writing, a reminiscence might be a good way to do so.