HVAC guys: Easy to change bedroom grills w/o dampers? See pics!

[Daniel R.]

Hello all:  Is there any logical explanation that many supply/return vents in my rooms (all 3 bedrooms and living room) don't have adjustments for me to close off hot air in the winter or cool air in the summer for the winter/summer months in rooms I don't use.  Especially in the cooler months to come, I would rather have warm air blowing in the rooms most commonly used.  

Are these ducts common to have in homes?  How easy is it to swap out new supply and returns? 

Thank you!

[JD Martin]

Also, if you can't find grills that fit you can buy magnetic covers that can adjust the amount of air coming out of the grill, and direct it into a different area.