Curious... recent versions of KDEnlive practically push this option into your face once you add a clip with properties different from the project.
If the option got disabled in your setup you can enable it again by going to
Configure KDEnlive ->
Misc (selected by default) ->
[ ] check if first added clip matches project profile
I am using this option in version 0.9.6 on Debian, but I've seen this behaviour in 0.9.4 and probably older versions as well.
Also if you got used at some point to dismissing this funny question window you get immediately when adding a clip, try to control the urge this time - the question is whether KDEnlive should adjust the project properties, which is what you want ;-)
(don't be mad at me if you think this last part is stupid, I've done user support occasionally and I have seen people do this;-)