Tumbler/Lock Assembly Removal

[MDF]

'84 300D. Last year the key sometimes would not turn in the tumbler, so I replaced the tumbler assembly with a new part from the dealer. The new part came with new keys. For over a year I thought the problem was solved. Now, the tumbler is stuck again. Really stuck. Tried all three keys (old key plus the two new ones).

I now realize that last year I should have replaced the lock assembly at the same time I replaced the tumbler. Is it possible to remove the lock assembly with the tumbler still in place? Or do I need to find a way to grind the tumbler collar off?

Ideas please?? Thanks-