An update: So last time, I simply didn't use any miles and went with the economy ticket.
But I'm facing the same dilemma again, because the company has asked me to move, and I'd really like to use my accumulated miles for the big move.
I guess I'll have to talk to the HR about what the different requirements are.
As before, the options seem to be:
1) Fly economy
2) Get upgradeable economy and upgrade to business
3) Book a separate economy reservation that is refundable and cancel the ticket, book a separate award booking (need to investigate whether the HR is okay with this)
4) Book a separate award booking reservation and get reimbursed for lowest advertised economy fare.
5) Book an award reservation and get reimbursed for the fuel surcharge + taxes
It'd be most straightforward if I could just get reimbursed for the lowest advertised price, but I'm not sure if that will fly.