In Rebels Sabine states her armor is 500 years old and she "re-forged" it to suit her. Boba likely did the same. As for the destroyed helmet, it could have been a fake, a decoy as such, or it too could have been reforged. Mandalorians reforging their ancestorial armor is nice cultural world building and a great story beat, and it covers the issue of screen changes and inconsistencies - win win.