In brief, you go to IPC 0. Your pay, however, will not drop, but will remain where it is until your Captain's IPC pay level exceeds your current rate of pay. For example. if your Lieutenant's IPC 7 pay falls between Captains IPC 2 and 3, you won't get a pay raise (except general cost of living increases, of course) until you are eligible for Captain IPC 3 based on time served in rank.