If anyone is doing the, "quit on the last checkpoint, then change the difficulty to hard." Here are all 13 of the checkpoints to quit out on to change the difficulty.
**May contain some spoilers. Read at own risk.
Chapter 1: When you're crawling up towards Optimus and Megatron.
Chapter 2: When you get off the artillery gun and state it needs more Energon, you start descending into an unknown territory.
Chapter 3: After your help is destroyed and you cross the bridge, before entering a door you have to pry open.
Chapter 4: When you have to escort the bomb.
Chapter 5: When you fight a couple of waves of Bug Bots, then there's an escape sequence. (When you transform into vehicle mode, quit here.)
Chapter 6: After you defeat the Titan, you will have to plant a bomb to escape.
Chapter 7: When you have to fight two Titans. (You will need to fight them on Hard, level ends after both are dead.)
Chapter 8: Right before you break into the control room. (As soon as you play as Bruticus, quit here.)
Chapter 9: After the arena wave battles that comprise this entire chapter, there's a save section to quit out at. (I quit out at this point, but when I loaded it back up after the wave battles. It made me fight the 5-6 Leapers again on Hard. Not bad since you have a Riot Cannon! and at this point you should have it fully upgraded.)
Chapter 10: When Soundwave is working at a computer to power up Trypticon and you have to defend him with help from a few turrets. (This sequence must be fought on Hard).
(*Make use of the Diffraction Barrier and the Thermo Mine Payload, they respawn again after a few seconds.)
Chapter 11: After the fight with Hardshell, there's a safe sequence to quit out before the level ends, before you cross the bridge, quit here.
Chapter 12: There is an escape sequence when Grimlock transforms into hes Dino mode and you have to escape before the time runs out.
Chapter 13: When you are asked to make your choice.