If the door is open (or can be opened, removing hinge pins, if possible, would be one way), the lock can be removed.
Profile cylinders and screw-in mortice cylinders have 1 screw in the edge of the door. Screw-in cylinders then unscrew, most profile cylinders then push out. If not, the plug needs to be turned - call a Master Locksmith http://www.locksmiths.co.uk/
Lock cylinders can be easily replaced - supplied with keys.
Rim cylinder latch/lock must be removed to access the cylinder fixing screws - then replace the cylinder.
Lever lock needs to be removed - undo screws in forend after removing handles if it is a 2-bolt lock. A master locksmith can rekey a secure modern lever lock with a new set of levers+keys, without needing a complete new lock.
Any other type of lock - call a master locksmith.
richard, December 2013
