Let’s assume a chain of events. We have a level six Tempest Cleric, who has access to Booming Blade through some combination of racial spell, feat, or multi-classing.
Further, also through feat or multiclassing, the character has picked up the Transmuted Spell metamagic.
When you cast a spell that deals a type of damage from the following list, you can spend 1 sorcery
point to change that damage type to one of the other listed types: acid, cold, fire, lightning, poison, thunder.
Booming Blade deals thunder damage, so it is eligible for this metamagic. Further, although it deals this damage twice, the wording of the metamagic does not limit it to only one roll, so we can assume it lets us replace both instances of damage with, in this case, lightning damage.
However, our character is a level six Tempest Cleric, and has access to the Thunderous Strike feature.
Thunderous Strike. At 6th level, when you deal lightning damage to a Large or smaller creature, you can also push it up to 10 feet away from you.
My question is, is there a reason the following wouldn’t work: We hit a creature with our lightning-transmuted Booming Blade, pushing it 10 feet away from us. On its own turn, the creature decides to close the distance – thus triggering the movement clause of Booming Blade, taking additional lightning damage and, because of Thunderous Strike, being pushed away again.