What kind of changes I can do in my website ?
Emphasis on a clean layout
A good website designer will ensure that the website that she or he designs will have a clean layout. Hence, in this way, the website will be quite easy to navigate and not something that people will have a tough time getting some useful information from. There are a ton of websites out there that have way too much graphics for it to be of any use. You wouldn't want your website to be something like this, as people are bound to get frustrated and stop visiting the site altogether.
Ability to change to your needs
Another simple way to ensure that you are in fact dealing with competent website designers is if they are able to deal with the kind of changes that you would want with the website. Hence, unless they are open to change, it is tough to get them to design the site like you would probably want it to be. There are quite a lot of things that might end up going wrong. But, if you are not able to get a designer that is willing to work with you on making these changes, it might not necessarily be a useful endeavor.