how does shifting the point of view [third person to first person] change the effect on the reader
It's very confusing. If a story or essay starts out in 3rd person, the reader picks up on that and expects that to continue -- a shift to another POV is confusing and could lead to not understanding the piece of writing.
it's not like changing the the pov in the middle of the story-- it's writing the same story in a whole new pov
It depends on how the change is effected. If the change is introduced clearly, it will not have a negative effect.
"Here is what one witness had to say...."
"This is John's version of what happened...."
The first person might be more personal, usually giving individual insights that might not be shared in a third person narrative.
I hope this helps a little more. Thanks for asking.