Okay so i am planning to get into a niche later down the road still working on my current project which i want to get to a part time on an then move into this new e commerce deal, but i want to get a head start an have some sites up ranking wordpress normal sites.

I want to know if i got a site to the first page of google for a competitive niche an it is in blog post format like normal wordpress sites out there can i turn it into a E commerce site an still keep my ranking will it mess up my SEO.

Ex. say i had a site like this dodocase's posterous - Home can i then turn it into this iPad Cases by DODOcase

I do understand they started with an E commerce an then added the blog but i don't have that yet should i just register a E commerce platform an spend that monthly bill to just have it or there will be no problem changing my blog to a e commerce without Effecting my SEO