It will surely happen. If you want the Spotify Devs to keep the songs if you change account, go to the Idea Submissions and submit an idea. Hope it helps.
Also, remember to clear data on android after changing accounts as Spotify keeps the song's data even if they are already removed. Spotify redownload's songs that are already downloaded and duplicates them as you log out and log in again (even with the same account). See the Android Settings for unusual storage use of the app.