If you hold a Y ticket and want to upgrade using skywards miles, you can only upgrade to the next class up. I.e. Y to J, J to F.

If you want to pay and upgrade then you can upgrade into either J or F from a Y ticket. It's the more expensive way around but it's still a way. IMO it's usually cheaper to just buy J or F in the first place as cash upgrading a Y ticket seems to cost more than just buying a J or F ticket.

I've paid 2560 dirhams in DXB to upgrade from Y to F one way to BOM. Whereas buying a F return DXB-BOM cost 4000 dirhams at that time.