Don’t keep it secret from the players, “I was actually a different class” isn’t as interesting as you think when it’s sprung out of the blue

Keep it secret from the characters, they don’t know what a wizard or a sorcerer or a warlock are, they just know that some people can do magic. It’s confusing for a stat block and not optimal for PCs, but what we would think of as sorcerer/wizard/warlock multiclasses are probably common in the actual dnd universe, someone with innate arcane power who made a deal with the devil to grow their power before studying it themselves or something, the lines are more blurred than strict classes and subclasses suggest.