When you connect from Intl to Domestic (or vv) on MU/FM, your domestic sector is booked in full price J cl bucket, which is eligible to a free upgrade (and which, if booked a standalone sector costs fortunes), subject to space. Typically, it can't be done until you are actually on the plane, and then you may be offered an upgrade proactively by the FA, or may need to ask them for it. If there is no space in J, you are probably the first candidate on the list for an op up.

I am not sure what happens if there are many people with full price J cl tickets wanting to upgrade, but I would guess its first come first served or status based.