Upgrading Prestashop - After some quick tips

[Flaunt]

Hi, I'm upgrading a client's website to the latest version of Prestashop. I wanted some quick advice on a few areas as I would like to be prepared for anything that may go wrong. The shop is an established and busy one so I can't afford any mistakes. I just have a couple of questions I'd like to clear up.

1) I'm upgrading from version 1.1.0.5 to 1.3.2.3. Is there anything in particular I can expect to go wrong with such a huge leap in versions? Will it even work properly? Where, most likely, can I expect problems?

2) Is there much chance their (heavily modified) theme will be compatible?

3) The Wiki gives instructions assuming your shop is stored on a subfolder (yoursite.com/shop). What's the best method if it's stored on the root folder?

4) Can I expect any database issues after upgrading?

Sorry if they may seem simple questions but I would just like to be 100% clear before I go ahead with it. Many thanks.