This is where you need to start:
CASA: Converting Overseas Licences
You don't need to give up your UK/EASA licence, but to keep it current you will need to either re-validate your EASA Class Rating(s) every 2 years (and you'll need an EASA instructor to fly an hour with - a CASA instructor won't do) or if you can't do that then you can
Renew by proficiency check when/if you return to the UK.