What happens if I play Dragon Form on a unit that has -1 [M] from Stupefy applied to it? Will my Fiora, Grand Duelist trigger? Is there a moment where my unit becomes 5 [M] before the -1 [M] from Stupefy brings it down to 4 [M]?

She will not trigger.

Although Dragon Form sets the base Might of a unit to 5 [M], it does so via layers. Layers are evaluated when the effect is applied and their evaluation includes all existing applied layer alterations. We process layers in order, starting from the Trait-Altering layer (where this base Might setting effect is applied), proceeding to the Ability-Altering layer (not part of this example), and proceeding to the Arithmetic layer (where Stupefy’s -1 [M] is applied), but this process happens instantaneously; there is no window while evaluating layers for the unit to see itself become 5 [M] and then go down to 4 [M]. As soon as the effect of Dragon Form is applied, the unit is 4 [M].

As a side note, Might is one of the traits that is managed in the Trait-Altering layer, and it is one of the copyable traits that copy effects can replace or add to.