no offense to anyone but 'on the morning' is not correct at all. that doesnt make sense in this case. if you were saying 'on the morning of... (eg that day)' then on makes sense. but in this case i have to say DEFINITELY 'in the morning'. definitely not 'on'

i think in this case they are both right but 'if i were' sounds like more formal and better english that would suit the lyrics better.