I get that they're trying to alleviate the overcrowding during breakfast (and to be clear, it is often very busy with long lines for tables) - this is not a new problem. For my recent 4-night stay at another property in Seoul, I chose to eat at the lounge every morning for the peace and quiet compare to the main restaurant, even though I had access to both.

However, (i) their initial email failed to convey the key point that could have lessened the impact of the change, i.e. that they have made significant improvements to the lounge breakfast and possibly preferable to the main restaurant, and (ii) in any case, it would have been better to highlight the improvements at the lounge and try to guide Diamonds there, but still give them the choice.