The last time I enquired with Arcam's US distributor, ASL Group, the upgrade cost was the same as the incremental difference between the cost of the new units. So, if yours had a list price of $750 for instance, and the upgraded unit listed for $1000, the price of the upgrade would be $250.
For a definative answer, contact the folks at Audiophile Systems Limited Group via the web site.
www.ASLgroup.com or http://www.aslgroup.com/audiophile/SupportConsumer.htm
Enjoy,
TIC