Restore

When i changed a backup  path (in Maintenance plan) from one disk(F - Drive) to Another disk (G - Drive), and trying to restore that time i noticed user name have been changed from SA to NT AUTHORITY\NETWORK SERVICES ,

1) Can you please explain why it happens ??

2)If i want to change a user name to SA (how can i do that)??

3)Does it effect a database ??

Thanks in Advance...