Does changing to a sub domain mess up SEO?

I was wondering if I should change my site (content) to a subdomain to indicate the type of service that is being offered. I would still keep the main domain as a landing page for customers but I will diversify in the future. There were 2 reasons for doing this.

1. I have a 5 letter domain name and I wish to use it as a brand and I wish to diversify in the future. In other words I will be using the home page as a landing page for all areas. I am concentrating on one particular market at the moment but I will almost certainly wish to expand.

2. The domain will possibly look better because it will show the marketing area, eg, healthyfood.example.com. (that is literally only an example market).

At the moment I am doing it all on the main domain but I have not advertised the site at all (because it is unfinished). All the content pages are indexed in google but you have to specify the domain name. The main questions is: Should I change it to a subdomain now or is it better (or OK) to change it later to a subdomain? It is a fairly complex site but it is just a case of deploying it with Visual Studio and copying over the database content.

I am happy to elaborate a bit more on request for any people eager to help me out!

P.S. I am not quite sure why but I am being a bit elusive as to what I am doing (it is not health food). When I am happier with how things are going I may be a bit more open with my website and the area of marketing I am in. It is nothing bad! I just do not feel too happy about elaborating too much. I was here about a year ago but I have had a bit of a delay.