This is a bad change by itself.
The Medusa head is a somewhat useful weapon PRE-hardmode with a very low chance to drop
it's definitely not OP, but it's useful for some things. example : beating bosses that spawn a lot of stuff like Ctuluh brain ....
it's definitely not OP, since it also misses targeting monsters with small hitboxes close to the ground ...
it's definetely not OP, since it has almost zero knockback & a long charge time leaving you vulnerable to attacks.
it's still somewhat useful in expert mode PRE-hardmode
however it's completely outclassed by other weapons such as : The Bee's Knees
therefore limiting the medusa monster to only hardmode means you're just pushing a fun somewhat useful weapons into the useless range.
you need to either reevaluate its amount of damage to make it more hardmode sensible, or its mana consumption, droprate, charging time... There's much much better weapons in hardmode accessible early on & that won't need as much farming as acquiring a medusa head requires (unless you're lucky of course, as is always the case with random ...)
Also I think it's fine to die from medusa in pre-hardmode, hardmode is already hard enough, without having to introduce new players with yet another new death mechanic imho.