Mercedes Repair Questions? Ask a Mechanic for Answers ASAP
First, have you tried spraying graphite into the cylinder, insert the key and tap on it with a hammer while trying to turn the key? If not, I would do this first as it can cure the sticking tumblers. If it will turn, then once a year, spray some graphite into the lock cylinder and insert the key and turn back and forth several times. This will prevent it from happening again.
If it doesnt work, then you can switch the tumblers from one cylinder to the other, provided they are the same diameter. But without seeing the inside of each, there is no way to tell you if they are interchangeable.