On many other cars, it's possible to set a new tumbler to your existing key. That would be the way to go.

If a new tumbler can't be set, see if someone will cut a key with the old pattern on one side, and the new pattern on the other side. Worst case is that you have to flip the key 180-degrees. Don't forget spares of this unique key. Now, MB tumblers might read both sides of the key. I doubt it, but it's possible.

The alarm shouldn't care about the tumbler as long as you get the right kind of tumbler. The alarm is set/unset by using the key, not by the presence of the key.

Sixto
91 300SE