Question: Can I level both my First Edition Fighters and my non-First Edition Fighters because they are different?
Answer: No, the only difference between the First Edition Fighter and non- First Edition Fighter is their artwork and other cosmetic features. Meaning, you can only level up one or the other. Only 1 of these characters can be placed on a team at a time (same as other Mythic Fighters).
Please take the time to double check which Mythic you are upgrading and if you received a First Edition Fighter and mistakenly leveled up the non-first edition, please open a ticket with customer service.