When I first set up Time Machine over the network, I realized that I didn't want the backup to happen using my regular user account, so that I could change my personal password at will. This hint quickly lists the steps you need to go through in order to make the change to an existing sparsebundle, so that you can keep doing incremental backups. It avoids the pain of a full backup after changing the user.
This setup assumes the following: Mac Pro with connected FW800 external drive and MacBook Pro laptop. Mac Pro backs up locally along with the MBP storing a sparsebundle over the network onto the same disk.
First, turn off Time Machine for the device associated with the sparsebundle you are changing. At a high level, the steps are:
Create a new user on the Mac Pro called backup
The first order of business is to create a new user from the Account Preferences screen. I created one with the name of Time Machine and used the username of backup. I also selected the option to make this account a "sharing-only" type, because I am not planning on using the account to login.
Recursively change the permissions on the sparsebundle to include "backup".
This is the tricky part. Yes, you can chown -R backup *sparsebundle on the sparsebundle you are working with, but then you lose the ability to look at it with your personal user account. So, I used the ACL facilities of chmod to add the new backup user into the mix.
$ cd /Volumes
$ sudo find Bob\'s\ MacBook\ Pro.sparsebundle -type d -exec chmod +a \
"backup allow read,write,list,add_file,search,add_subdirectory,readattr,writeattr,readextattr,writeextattr,readsecurity" {} \;
$ sudo find Bob\'s\ MacBook\ Pro.sparsebundle -type f -exec chmod +a \
"backup allow read,write,append,readattr,writeattr,readextattr,writeextattr,readsecurity" {} \;Mac OS X Hints
http://hints.macworld.com/article.php?story=20091129044531695