Based on the fact that all of the past Transformers movies have been rated PG-13 I doubt that this one will be rated differently. Even though the new movie does look more violent I doubt the rating will be effected.
Transformers: Dark of the Moon (2011) is rated PG-13.
3 stars
Transformers 2 has been rated as a 3 star movie
There is no official rating. But is most likely a PG-13 rating
Transformers - 1984 Webworld 3-16 is rated/received certificates of: Australia:G
Transformers - 1984 Chaos 3-7 is rated/received certificates of: Australia:G
Rambo 3 (1988) is rated R.
Transformers - 1984 Surprise Party 3-12 is rated/received certificates of: Australia:G
Transformers - 1984 Call of the Primitives 3-26 is rated/received certificates of: Australia:G
Transformers - 1984 The Dweller in the Depths 3-18 is rated/received certificates of: Australia:G
Transformers - 1984 Ghost in the Machine 3-17 is rated/received certificates of: Australia:G
Transformers - 1984 Fight or Flee 3-15 is rated/received certificates of: Australia:G