No, I'm sure you can use your pre-JAR one. I'd double check with CAA FCLD, you may well save yourself some beer tokens. May be wrong though - please let me know if I am
From LASORS:
A6 FLIGHT CREW LICENCES (JAR
AND UK NATIONAL)
Flight crew licences issued in accordance with
JAR–FCL will conform to JAR-FCL specifications and
may only be replaced with a JAR-FCL licence. Licences
issued in accordance with UK National arrangements
will, where applicable, remain valid until their date of
expiry and may be renewed after this date. The UK
National Licence will continue to be available to those
who have previously held the licence. There is no
necessity for UK national licence holders to convert to
a JAR-FCL equivalent licence unless they wish to do
so. The only circumstance in which all holders of a UK
national licence would need to obtain a JAR-FCL
licence would be if the EU mandated such a move. Until
such time as EC Directive 91/670/EEC is withdrawn, a
UK National licence can still be validated in other EU
States.Holders of UK national licences do not need to
surrender their licence upon issuance of a JAR-FCL
equivalent.