[business_analysis] Hosting: let users change their products — instance

_Automatically composed from the parent epic at dispatch time._

Settled the "change hosting product" scope. It covers live storage growth, an instant in-place type change, and a confirmed reprovision for incompatible types, with billing that ends the old product and starts the new one prorated and invoiced now. The PRD has 20 acceptance criteria, each with a dev-environment oracle. Assumption: charge or credit immediately (DEC-4).