Forget what you know about the medical mart. Very quietly, the project is turning into something very different than what Cuyahoga County commissioners promised years ago when they imposed a higher sales tax to pay for it. No longer is the mart envisioned exclusively as a first-class superstore for doctors and other health care professionals eager to buy the latest in medical equipment, furniture and supplies. Instead, private developer MMPI has in recent weeks shown a shift toward tenants who would provide educational services rather than sell expensive products. The Chicago company has done so without fully explaining its thinking to the taxpayers footing the bill. Meanwhile, public money continues to flow to the $465 million development, which will include a new convention center that officials hope will draw hundreds of thousands of visitors to downtown Cleveland. And even the most ardent supporters are scratching their heads.