If you already hold a EASA PPL(A) and are looking to obtain a EASA LAPL(A) then this is a "re-grade" in CAA speak, so you will need CAA form SRG1190:
http://www.caa.co.uk/docs/33/SRG1190FFenabled1.pdf
You would have to hand back your PPL in order to obtain another licence (i.e. LAPL) for that category of aircraft.
NPA 2014-29 has proposed that LAPL privileges are embedded within a PPL but this amendment to EASA regulation is neither confirmed nor implemented.
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