[Lonestar]

Currently, there's a few parts that alter the damage type but never been a balance issue before:
Hellhound Bullets (Pierce > Fire)
Holy Arrows (Pierce > Light)
Fire Arrows (Pierce > Fire)
Spiked Knuckles (Blunt > Pierce)
I think that's the only ones that I remember, but they've never been a problem before, so I guess it would be okay to change Piercing Blade as well.
Plus, Piercing has two downsides which is -5 crit/hit for +2 power, so I guess the damage type change compliments it well.

[MegaBlues]

Generally, elemental damage is worse than physical. So going from physical to elemental or swapping between elemental or physical damage types shouldn't cause any issues.

[Snake]

i LOVE thIS.