It doesn't sound like too drastic a change, in text, but looking through the examples on rob!'s blog, you can see how it really takes the zing out of the character's visual impact, and how the new design was, apparently (and surprisingly), no less difficult for more modern artists to draw.
But PS may have been a challenge even if he'd remained in a cape, although I can't help but think rendering his suit in solid blacks would have hidden a lot of poor attempts at drawing real-world mens' formal fashion. In particular, few artists of the 90's onward seem capable of drawing convincing mens' hats. The early PS artists of the 60's and 70's (Aparo, Neal Adams, Bill Draut) all had a good grasp on drawing street clothes, not in small part, I am sure, to their experiences in advertising and in the ubiquity of civilian garb in comics of that era. And of course, the fact that mens' dress hats were actually still in style when they learned their craft.
I made the claim that Jim's version is the definitive one. Although Jim's version followed Neal Adams' redesign very closely (the turtleneck, the medallion, the collared cape), it was a single daring innovation that Aparo contributed that made all the difference to me.
When Bill Draut began drawing the 60's revival, he emphasised shadows over the Stranger's eyes, although he still frequently showed those eyes, inside the shadow. Adams adopted the convention of drawing the character's eyes as solid white shapes, glowing from the shadows of his hat brim, much like the Spectre's eyes glowed under his cowl.
Jim continued this approach for a while (and continued it on the covers), but after a few issues, he took it a step further: the shadows swallowed the eyes entirely in an inky blackness. No longer could readers misconstrue (as some have admitted) that the Phantom Stranger was wearing a domino mask under his hat. Instead, there was that eerie, eternal night cast over his face. The conceit may have seemed unlikely to some readers, but I found it haunting, and richly symbolic of the Stranger's mystery and ultimate unknowability. No matter how hard we try, we are prevented from connecting with this enigmatic hero on the most instinctively direct level.
I called this move "daring", and it was. Consider: when Jim began doing this, writer Len Wein had begun humanizing the character, giving him a love interest in Cassandra Craft, with more emotion-charged plots and physical danger. And in the midst of this, Jim Aparo willing gives up one of the artist's primary means of conveying emotion: the eyes!
And yet Jim never appeared to struggle with that choice. The Stranger's emotions, when on display, were unmistakeable, and in those trademark moments of inscrutible grimness, those blackened eyes made the images all the more effective. Even when the Stranger would lose his hat, Jim found ways to make the unlikely shadows still work.
But not all artists are up to the challenges Jim could tackle, and as others took on the character, they invariably fell back to using the easy glowing white eyes (or, more disappointing, actually showing the character's eyes: I recall being hugely disappointed in Romeo Tanghal for giving us a straight-on look under the brim in a DC Superstars that came out soon after the cancellation of the Stranger's own comic.
And so I tip my hat to Jim Aparo for his visual definition of one of my favorite characters. Only I'm tipping my hat down, so you can't see my eyes.