I asked for the bits in our very own Wanted section but got nothing back.
I spoke to my local dealer (Vines of Guildford) and the bits come to approx. £100 after you've bought the seat.
Why would you want to change, most people seem to want to go the other way, changing from GSA seat to GS. You could try asking if anyone wants to swap parts. Though I guess most would want to keep both, for when it comes round to selling.