All I did was wrote "Continued in next logbook" across the last page, and then transferred my cumulative totals across to the summary page at the beginning of the CPL one. I have kept the PPL logbook, of course.
Whilst you're doing the totting up for the summary, take the time to divide it my aircraft type, Solo/P1S/PuT and local vs cross country as you will have to put those on the CAA forms when you apply for your Commercial licence and it will save a lot of hassle then.