I'm afraid not. Altrough i'd suggest to plarium to allow "better" heroes to be used for fusions (like, if you have a 6* and you only need a 5* you can use the 6*), at the time being your marshal cant be used for the fusion anymore, and there is no way to reverse that.

Actually, in rare acts of display of humanity, plarium has been known to de-level heroes for fusion purposes.

My advice is just fill a ticket to them explaining what happened and hope they actually read it. If they DO read it, they will help you.

Thanks, hopefully they hear my plea

I heard a rumor they're gonna fix this in 1.12.

That sounds fun actually! They should let us pay like 50 gems or something to reset champs- like say my Level 60 Kael Ascend lvl 6 back to 3 stars And lv 0 Ascension, and give us Chickens equal to how much we need to get em back to lv 60 6 star, And all the potions! Obv we still gotta level em, but this would b fun! 

Just an idea- no hate plz