Most states' lost comm procedures more or less say to maintain the higher of either cleared altitude, altitude to be expected, when expected (when given by a clearance e.g. "maintain FL200 expect FL230 in 10 minutes"), or the MEA / minimum instrument altitude for the route segment. Changing altitude to abide by hemispherical rules, and changing altitude to follow a flight plan when you have not previously been actually CLEARED to that altitude, are not part of it in most cases.