Sometimes we need to tell the backstory of our characters.
What made them the person they are today?
What motivates them on their journey?
There are a variety of ways to give clues to a time change -- or even a dream sequence. The film can turn black and white, go into "old time vintage movie mode" if you have that option in your editor, sepia tones? Sure.
Anything to give a hint to change of place or time -- or both will alert the viewer that their are gears being changed.
I was watching an old TV show this evening. Lots of action and too much killing for my personal tastes, but I was looking with a directors eye. And what I saw was interesting. Instead of "memories" become faded or dim -- in this case they became supersaturated? It was as if the memories held more power than the current reality. And indeed, I suspect that was what the director was trying to impart.
So when you are faced with a time shift, it might be good to remember that memories aren't always faded. They can be fast and furious. They can push us over the edge as needed for the storyline. ANY effects change that alerts the viewer to a different timeline could be affective -- it just depends on your story.