Hi
I often upgrade packages via apt-get.
After I decided to give higher priority to packages from extras, apt-get wants to downgrade e.g. kernel-power.
Is it possible to force apt-get to leave newer packages that are already installed?
thanks in advance

Yes, remove the pinning!

Really, apt-pinning is some science of its own. If you are use repositories with a couple of versions like it is with kernel-power, your best option might be looking into holding of packages instead of pinning packages to repositories.
You might pin your extras repo higher, so that versions of testing and devel might be installed with the -t option, but would not get touched during normal upgrade.