There is no MI case specifically on point about the transition from OC to CC in a vehicle, but MSP Legal Update 86 states the following advice for its officers:
"There is no way to “open carry” a pistol in a vehicle. An individual, without a CPL or otherwise exempted (e.g., a police officer), who transports a pistol in a vehicle to an area where he or she intends to “open carry”
may be in violation of MCL 750.227."
"MCL 750.227(2) makes it a felony for a person to transport a pistol anywhere in a vehicle unless the person is licensed to carry a concealed pistol. Exceptions to the above statute are found in MCL 750.231a."
http://miopencarry.org/sites/default/files/MSP_Legal_Update_No._86_2_336854_7.pdfI read in a MI case, though, that "concealed" means "not easily seen upon casual observation." I'm paraphrasing here, but you get the point -- once I'm in my car, my otherwise OC'd pistol is no longer seen by casual observers and, thus, it's "concealed."
