update shop

Hi, I have the following question. I am a new customer and for the time being quite happy with my subscription in omegatheme.com. My question is:
1. what happens when I try to update my shop version, lets say prestashop 1.5.4.1 -> 1.5.5. I see there have been made changes to class files, and therefore, using the module 1-click update, some of the classes will be changed and errors will show.
2. What is the difference between certified prestashop companies and not certified prestashop companies. Does it mean that certified prestashop companies create themes without editing clases and modules files and then updated will not affect the snop, and changing different themes will also be possible.
3. If I try to use multistore function on prestashop and for different shop use different themes, will this be possible, as long as classes and modules for different templates are not the same.

Hello,

Sorry for late response cause we're upgrading prestashop theme. I hope our theme will be released new version 1.5.5 soon

Regards