If they are beyond the incontestability period it should not matter.  The rates are set based on age and gender characteristics  changing one's gender is superficial and should not change the mortality for the individual.  If they started out as male going to female should not give them a lower rating and vice versa a female going to male should not get a higher rating.  Each company out there may approach the issue differently so you need to check with each carrier.  But disclosure is always paramount there is no reason to give them an excuse to try and negate a policy.