What is involved in the upgrade ... is it just a matter of replacing the base, besides the pump ?
Just base and Pump is required
Is the pump complete, or are old parts re-used ?
The PSK1000 pump is supplied complete with new volute
And hows availability for replacement parts ?
Excellent, unlike Eden, all Sicce spares are available through us and they have a base in the USA (Sicce USA)
And, of course, what's the upgrade cost ?
Not sure yet, but affordable
I will stay away from a Sicce as no local to me vendors carry them.
There are NO Eden dealers in the whole of North America, but you have Sicce available right here 
any local dealer has access to spares
Of course there is no reason to upgrade all the time you are happy with the Eden pump, but for those that wish to upgrade to gain the performance jump they will have an option to do so