Changing POV style mid-story
I'm writing a story that is told by the main character. The idea is that at some point, he decides that what has happened merits documenting, so he's telling part of it in the past tense. The problem is that I want him to develop into an unreliable narrator as the story progresses into the future, so I need to have him eventually "catch up" to "now" and start documenting events as each day progresses.
To keep it sounding believable, that means the POV/tense will have to shift a bit, from narrating a series of events in the past, to writing what amount to "journal updates" in the present/near-past. One has the benefit of perspective, the other does not. For example:
Little did I know what would be the consequences of my decision...
I just had to keep on walking and hope nobody noticed...
Two descriptions of the same scene, but one has hindsight while the other is blind. Will that change-over mid-way through the story be jarring or distracting? Should I rework it so I don't have to switch perspectives?
TL;DR: Will switching perspectives from past to present partway through a story cause problems?