Do I need to change the circuit breaker for a new stove with a lighter cord?
Like stated in the comment, appliances only draw the current they need. So simply put, you're fine. It sounds like new stove uses less power than the old one. 

Edit since I feel like I should explain better...
The way it works, is a 50-amp receptacle has the ability to supply 50 amps of current. It is the device that controls how much it uses. Operating normally, your stove won't use more than 40 amps of current. Chances are it will normally use less than that.