It depends on the engine type and reason for reversion to Alternate mode.
IF the EEC was happy and healthy and is only in Hard Alternate due to manual selection of the EEC mode switch, it can be returned to Normal mode at any time by selecting the EEC mode switch back to Normal.
On the EPR engines (Rolls/Pratt), if the reversion was due to loss of inlet probe heat, Normal mode cannot be re-selected without shutting down and restarting the engine even if probe heat has been restored.
On GE, if the EEC went to alternate mode due to a temporary loss of Air Data (ADIRU and/or SAARU) or air data disagree, once normal air data has been restored Normal mode can be reselected.
Clear?