Nothing is confirmed yet. But you can use past examples such as the takeover of Swiss to be fairly confident that diamond club won't be forever.

However, the change won't be instant, there'll probably be at least a year more of DC. However, there's nothing to stop them changing the rules of DC in order to integrate better with M&M.

KLM did something similar. Flying Dutchman used to earn 100% miles in all economy fare classes but a few months before the switch to Flying Blue they introduced 0 miles on the two lowest fare classes. This meant when Flying Blue was introduced the 25% miles offering for the two cheapest classes would look like a genuine enhancement.