You can switch hosting & payment plans at any time through the control panel.
You can ask sales/support, but I don't think they'll refund a setup fee if you start out monthly, then upgrade later. If you use a promo-code, you wouldn't really be paying a setup-fee anyway, though.
I forget how they worked the discount when switching yearly/2-year when I did it. I don't remember getting mad or smashing anything, so I think it must have been a pretty fair process.
I think they gave me the two year discount price, then credited me for anything I had already paid, then just billed me the difference.
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