To the op:The effect of celtic guard would still activate even though Leotaur original attack is 1500, making it 2000. Therefore leo shouldn't 🔥 Celtic theoretically. I never tried myself although, will update here after doing so. Ignore those rude folks who give rogue answer. They are probably some insecured bad kids.
OP: leataur will not 🔥 this card. Reasons is because if this card battles a monster with 1900 at or more, it will not be destroyed. Not if this card battles a monster with original at of 1900 or more. Otherwise, then leataur would 🔥 obnoxious Celtics guard.
Who need proof bruh? If Ob Celtic Guard can't be killed by monster over 1900 ORIGINAL atk, then any simple equip increasing atk can 🔥 the guard, but it ISN'T. If the ATK of a monster increase to 1900 or more, Ob Celtic Guard can't be killed anymore.