TL;DR The following spell wants to be used in my current game, should I approve it or is it a little bit too broken/exploitable?

I know it might be a opinion based question so try to give an argument as to why? More than just something along the lines of *it sounds ok to me*.

> **Eldritch Mutation**
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> *Prerequisite: Pact of Chain:*
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> You can use a warlock spell slot to cast a spell that will transform your familiar into a creature of higher power.
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> The creature it turns into needs to be of the same Race/Type as the *Familiar*. But with a Challenge Rating that is equal to your warlock level divided by half and rounded up.
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> While the Familiar is transformed it has its own initiative and it acts on its own. It's friendly towards you and your allies. However it does follow your commands (command takes bonus action). 
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> The transformations last an hour and it takes concentration to maintain the spell, you can use a bonus action to finish the spell.
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> If concentration ends the familiar will go hostile towards everything on sight.
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>The familiar will go back to normal if its hit points hit 0 or after an hour has passed, even if you lost concentration.