My site is currently default without the www at the start of the domain. It looks unprofessional to me, and I want to make it have the www. But this will change my SERPs wont it? Because all my pages in Google don't have the www.
I think now you should continue promoting your site without using www as it may harm your search engine ranking
There is no good reason to change your site to www. as some popular site like digg.com also don't have www prefix.