While I'm not 100% up to date on Ant-Man, I do believe that Scott Lang becomes Ant-Man when Pym retires. I've only read a few of the comics and Lang was Ant-Man in them.
Where they veer off into there own thing is having Yellowjacket as the villian. I thought he was another incarnation of the Ant-Man suit, and a good guy.
I don't mind when they switch the scripts around if it makes the movies better, but too many Marvel villians lately are just mirror copies of the heroes. They need to get away from this and start bringing in some unique bad guys with different powers and abilities or it's going to get stale real quick.