There's the "Saving changes for existing table design is now required.
Do you want to save the design now?

Warning! Any data in this table will be removed upon design's saving!"

Improvements:

• better button texts, "Save" and "Discard" were apparently not enough for some users, see e.g. https://forum.kde.org/viewtopic.php?f=220&t=131143&p=351981#p351973
• Fix the issue with displaying HTML tags due to joining parts of messages, now we're using KLocalizedString more for lazy evaluation

Old:

Create a test table with at least 1 record, add/remove one column from the design, switch to the data view. Observe the message.