In theory, a conversion is certainly possible, but it's probably more cost-effective to trade in your old one for a new one... they've caused a huge extra expense!
see also here /viewtopic.php?t=3201
and here /viewtopic.php?t=3176
If that's the case, you won't save anything because D3 and D4 are taxed the same way. D4 only has a one-time tax bonus of €614! But I don't think that will be revoked later.