Class Morph

This spell temporarily changes the class of any humanoid creature that currently has one But it will keep their level the same. An unwilling humanoid must make a Wisdom saving throw to avoid the effect. The spell has no effect if the creature is divine in nature or has a intelligence of 6 or lower. 

The spell lasts for the duration, or until the target chooses to drop the spell. A creature who you reform can choose the class themselves if they inform you before the spell begins, additionally their level transfers allowing them the benefits of any subclass or feats they choose.

The humanoid is limited to the actions it can perform based on it's new class. (A prior wizard can't cast spells, a barbarian can't rage, etc.)  The new class can be customized to suit the person taking it if wanted through creation of a new character sheet or by one you make for them.

Casting this spell on a hostile creature causes the spell to become an instant cast but it automatically prompts three skill checks, the first pertaining to the wisdom save needed, the second being if they get the class they want or if you choose it for them (a contested roll with your spell casting DC or Wisdom if a creature doesn't normally cast), and the third to tell how much time they will remain as the class (Using a Dice 20 you roll for how many seconds in increments of 3 per 1 on the die, meaning a 20 on the die is equal to 1 minute or 10 rounds of combat, this roll is done without any added modifiers, inspiration, or advantage except under the wish spell. ( That's something you should take up with your DM, this is just the guidelines.)