SRG1104 to be used in both cases, although it's a bit of a fudge how you fill it in as the form is technically for conversion of non-EASA licences...
The correct form for re-grade of a licence from EASA PPL to LAPL or vice-versa is SRG1190. Surprised the CAA don't know that.
SRG1190: Application for the Re-Grade of UK Issued Pilots Licences to Part-FCL, Private Pilot?s Licences, Light Aircraft Pilot Licences or National Private Pilot Licences
ifitaint..