Don't even whisper an idea like that near the CAA!
The Raven is regarded as part of the R44 type. I believe that JAR-FCL does not regard it as a variant so formal difference training is not required. Do go and get some familiarisation training though.
Having said that most of what you need to know is in the POH/Flight Manual.
What you need is a ground review of the changes in the Raven, and a short training flight including simulated hydraulic failures. It would be worth getting your instructor to annotate the flight entry in your log book to confirm you had received the training. Many schools will want to see this, before hiring to you.
It does fly much better than the Astro.