Re-equipping monsters to a ranked up monster
Would my equip monster be destroyed or would it stay on the field attached to the new utopia on the field?


Say you got some card, like ZW - Leo Arms, attached to your Number 39: Utopia. 

If you rank-up your Utopia, it becomes an XYZ material of the new card you rank-up summoned. In this case, as Utopia is no longer face-up on the field, Leo Arms would then "lose" its target, and be destroyed and sent to the graveyard (or wherever it should go).

From the Equip cards wikia page:


  If the equipped monster is no longer face-up on the field, or is no longer a valid target, the Equip Card is immediately destroyed.