OBD2 systems "learn" their settings. The system adjusts ( Integrator and block learn) and if the system is totally reset it takes a "drive cycle" to relearn. Skipping the drive cycle will sometimes throw a code saying the engine is too rich or too lean - I believe they are
1130 codes. Changing to a radically different fuel will require a change larger than the integrator can handle "in one sitting" Untill the system has "learned" the new fuel, you can have problems (CEL on with P1130, 1131, 1132 1137 or 1138 codes) Going to E85 too fast would likely throw an 1131 or 1137 code (engine lean beyond adaptive limit, or something like that)