I agree, to a point. There are some aspects of a character's change that are easy to make without totally revamping the overall character, but there are also some that you just have to alter in order for the character to make sense in their new role.
In today's wrestling, I feel like it's easier to change someone over from heel to face or vice versa without much change, mainly because the characters themselves aren't so pointed to one side anymore. The whole "heel" and "face" mind frame is blurred, and has been since the Attitude Era. When fans started cheering for guys like DX, Stone Cold and The Rock, who all exhibited characteristics that were heelish in nature, that's when you really began to blur the line to the point where you are today. The world was becoming a gritter place at that time, more grey that black and white. People were angry and wanted someone to be their proxy in taking out aggression. Hence, Stone Cold's face turn with the same attitude was monstrous. For The Rock, it was turning him at the right time. His persona was larger than life, with people latching on to the catchphrases. That allowed him to shift without much change, like he's done each time he's flipped. For DX, it was their don't-give-a-damn attitude against the establishment and the popularity of that movement which made their flip work.
It also depends on who that character is in a program with and where they want to bring that character in the long term. For example, Edge's "Rated R Superstar" character was a great heel, but was toned down significantly when he turned face and it worked because he faced bigger heels. Orton's changes weren't a total attitude shift, just some minor tweaks and being double-crossed by heels did it. Same with Punk. He hasn't changed his brash attitude. Now, he just focuses it on the most hated face in the world (Cena) and the heel establishment (Vince, JL), similar to what DX and Austin did in the '90s. Actually, you can probably trace this trend back to as far as Roddy Piper back in the late 1980 and 1990s. He was an absolute asshole, but when you put him against a bigger heel, he came off as the face anyway without changing a thing about him. Same with Ric Flair during his face runs.
About changing some of the character's catchphrases and such, that's just part of the business nowadays. Merchandising is a big business, so any time they flip a character or just freshen them up a bit, it will usually mean a new direction in order to sell different t-shirts, hats, etc. Think about it. If a character is now trying to play up to the crowd and not draw heat, you may have to change their catchphrases or whatever to get the character over with their new attitude so that people will buy their stuff. For example, it's hard (but not impossible) to get a heel turning face over when the guy/girl keeps a "I'm better than you" attitude and message.
It's true that some characters have been flipped when they maybe didn't have to be. Sometimes, though, it's needed when a major star either leaves the company or is gone for a long time because of injury. Someone needs to fill that void, which opens up the opportunity to freshen up a character by turning them or by giving an up-and-comer a chance by flipping them and putting them in a main event program. Plus, the fans - not just the wrestlers and brass - get tired of some characters and their shtick after a while, which calls for a change. You can't keep the same thing going forever.
Basically, while some gimmicks and personas don't need much work to flip anymore, it's not a gimmie to just flip someone and keep them the same every time. Sometimes, change is good and necessary to keep the business from getting boring and predictable