The SEP remains unaffected.
In fact, they won't even print it on your CPL licence page, you will have to take your PPL ratings page and stick it in your CPL.
The SEP is revalidated in the second year, so if it's in that period you could get the examiner to sign your ratings page once you have passed. It will keep the same reval date, but be valid for a further two years.
The bureaucracy of renewing an SEP is minimal, provided you have the experience you just have to get an examiner to sign your ratings page and send a one side of A4 form to the CAA with no fee. No need to send licence or logbook, just the form.
If you don't have the experience, you do an LPC with an examiner, and he signs your licence when he's happy.
Also, complex SEP is no different to simple SEP, you could get complex differences training signed off as a PPL, there is no difference in the rating.
Last edited by RTN11; 2nd May 2013 at 17:16.