alter table comments convert to character set utf8mb4;
Was a little harder for posts (threads) because the titles are indexed, and index keys may not be longer than 1000 bytes. Titles were 255 chars max, but when you use utf8mb4 you are using 4 characters per byte, and 255 * 4 = 1020 bytes. So I had to reduce max title length to 250 for posts, but that didn't affect any existing titles.
#patnet