The tranny has a fluid temperature sensor in it that tells the PCM to modify the line pressure and shift points to compensate for cold temps. Once the fluid is up to temp, the standard strategy is in force. The sensor also keeps the torque converter clutch locked out until the fluid is warmed up.
Your best bet is to have the tranny serviced by a good shop where the fluid in the torque converter is drained and the system refilled with Mercon V or ATF+4 (I'd go with the ATF+4) with a new filter and pan gasket.
Changing the filter will not cause any tranny problems.
Flushing the tranny sometimes causes some troubles, so I would avoid having that service performed even if an "expert" recommends the procedure.