We had a HN thread a few weeks back on the K-9 Donations blog, basically the K-9 Play version is still the old client UI (but the betas are not 100% feature complete); you can opt-in to the K-9 beta on Play or F-Droid and get the new UI experience if some features like IMAP IDLE are not that important to you. (side comment, the new K-9 betas allow export/import of settings but not sure about the older version you're currently using)