Why would only Steven's change (Corley to Coleman) get that explanation? I would have probably expected it for Adam (Thomson to Sachs), Fallon (Martin to Samms) and Amanda (Oxenberg to Cellini) as well.
In the story, it was written that Steven had been involved in an oil rig explosion at sea. When it was decided by the producers to finally recast the character after Al Corley had refused multiple offers to return, the explanation was given that Steven had to undergo plastic surgery to reconstruct his face after suffering severe burns. This made way for an explanation that Steven would look different upon his eventual return to the show.
Although Jack Coleman never compared to Al Corley, I did feel he was quite serviceable as Steven, and I did warm up to him rather quickly.
The one that had the most dramatic transformation was Fallon. When examining her, it's like taking a look at two entirely different characters, naturally played by two entirely different actresses. In the first four seasons, Fallon (as played by Pamela Sue Martin) was Blake's headstrong, temperamental daughter who had business aspirations. She hadn't any ideas to conforming herself to any man or to any image someone had of her other than her own. When Fallon returns in Season Five (now played by Emma Samms), she is now the exact opposite---she claims she's conformed to the image her father and Jeff have put on her, and that she's allowed them to protect her by being their little girl. Totally out of character when in comparison to the original's bite, but at the same time it somehow fits for this "New Fallon" and her relationship with the same ole Jeff.
Adam was brought in as a stand-in for the absent Steven. Gordon Thomson did an excellent job playing him, and I haven't seen the reunion miniseries as of yet, so I cannot comment on the actor that replaced him in it.
As for Amanda, I always saw her as a Fallon filler. Ironically, however, she wasn't really a filler for Martin's Fallon, but a serviceable stand-in for Samms' eventual Fallon. Amanda came in, as I understand it, amid much fanfare. Aside from sleeping with her mother's husband and having her wedding invaded by terrorists, however, she had little more to do than mop around and whine. And that got old. Quick, too. Catherine Oxenberg was certainly a beautiful young lady, and I'm sure the producers did everything they could to promote her as their imitation of Lady Diana, but they never seemed to get it quite right with her. She was axed because she wanted a raise. I've always wondered why they decided to replace her with Karen Cellini briefly, when they could have just as easily let Amanda die in that fire with Claudia.