Welcome aboard!
I personally have found some folks feel it may hurt value, as PSA may have had looser grading standards at the time. So, many people have had their cards in those old flips re-holdered to the modern flip and case.
Here's one example - identical cards (PSA 10 Marino rookie) sold by the same seller, ending one minute apart. (Mine was the old flip, and I didn't shill it, but I don't think that was the issue here!)
Old Flip
New Flip
Price was $150 apart ($1480 vs. $1630), with the old flip selling for less. I've seen many examples of similar, so I think people may seek to re-holder those older flips to get away from the perception that grading standards may have been weaker in the beginning.