I’m only speaking generally:
the 3GPP version of a MS is not upgradeable later on. You’ll have to buy a v8 phone when the manufacturers release them.
What is the fast dormancy feature?
If it was initially designed in 3GPP version 5, but is finalized in v8, then there is a possibility that *some* v5 MS can support it.
If it appears from scratch in v8, then v5 MS can’t support it.