There are lots of scenarios for your situation.
1. You don't have to worry about OPT because I assume you used your OPT more than a year. IF "yes" you are no longer eligible for OPT since you used it for more than a year.
2. You have to maintain your status(CPT if you want to work even if your H1B is approved) If not you can start working once your H1B gets approved.
3. Your employer will request for change of status but it depends the situation because USCIS may ask for your college documents as an additional proof during your H1B process and there might be chances where they might deny your change of status and approve your H1B, so in that case you have to continue your CPT or get the stamping from somewhere else to get your change of status approved.
4. If your H1B gets approved then you have to contact your adviser from your school to get your status updated. In that case you will no longer be on CPT/F1, You will be moved to H1b status.

Just sharing my experience. Let me know if I confused you.