Scenario:
Aircraft gets airborne. SOP is for TWR to do a Mode 'C' validation before transfer to the centre. North bounds go to one centre, South bounds go to another.
West European heavy aircraft gets airborne, ATCO asks for a level check - nothing. Calls again - nothing. Aircraft re-appears on frequency having self transferred to the NORTHERN frequency when he was in fact SOUTHBOUND. (Obviously scratching of heads at the north bound area centre as they weren't expecting him!).
Don't change unless given a positive instruction to change please - be it at the time or as part of a clearance.