Rather than just port across the cartridge original (in the shovelware style that MegaCD game releases users were getting used to).
Though the original was well received, the CD version featured all new graphics & levels in a larger & less linear world. There was also an original score, as well as the obligatory Mega CD video clips.
The new music was by Tommy Tallarico, some truly haunting tunes. It’s considered some of his best work.
The game game squeezed the extra power from the new processor (clocking it from 7 to 12mhz) making a much better game than the original.
The fact remains that Sega really should have taken the opportunity also updating the graphics chip, there were rumours, but they stayed just that. Where the Super Nintendo could choose from a palette of 32000 colours, the Mega CD still only 512..