According to Jai, the reshoots are to add more action:
The humour has been there from the first trailer, but the last two have more, that's all. With characters like Harley and the Joker, the humour was always going to be there no matter what. The three trailers have been good, I prefer the first two though.
What happened to the people whining about the tattoos? Nothing to say anymore? They love everything Ayer is doing now?

t:
The movie should get decent reviews because it should be fun enough for shallow critics, but some of David Ayer's movies have bad reviews:
Sabotage RT 19%
Street Kings RT 36%
There are no guarantees, but if I like the movie, that's what matters to me, I don't care about critics that seem to think all these movies should have the same tone or forced, dumb juvenile humour to work. Nolan was lucky they didn't care back then, Tim Burton was not so lucky with Batman Returns:
So, now dark is out again and fun is in... till critics change their minds again!! That's how people work, they just get tired of one thing after a while, it doesn't matter how good or not the object actually is. People are fickle!