How to change dm_crypt aes/256 LVM password after install?

[johhny_at_poland77]

I installed a Fedora 14 & an Ubuntu 10.04 LTS on my PC.
I used dm_crypt/LVM/with aes256 in both install.
(2 VolumeGroups, each encrypted, so there is 2 separated & encrypted linux distro on this machine) 
The Question: How can i change the passwords for the encrypted VG's?

Thank you in anticipation!