Transpositionist

When an [Empowered] Gangplank, Naval replaces Transpositionist's return, you may still play a unit that has less might than Gangplank's current might after the +3 increase applies, provided that unit is in your hand or chosen champion zone or a separate effect permits you to play it from another zone.^{420.1.a420.3.b}

Replacing Gangplank's return does not stop Transpositionist's next instruction because it refers to Gangplank and his might, not whether the return happened.^{359.3.e.14.a359.3.e.14.b407.5} Gangplank remains on the board and remains a legal target, so the ability can still read his might.^359.3.e.12.a It reads that value when the later instruction executes, after the +3 increase applies.^359.3.f.2

An otherwise unmodified Gangplank has 9 at that point, so the unit must have less than 9.