Ethics of web design

Hi

Im in the process of doing some work for a site I designed and built a few years ago, but this time around im wanting to put it into word press so that the client can manage certain aspects i.e pricing, phone numbers etc without editing the actual file.

Because im not very experienced in word press I was planning on using the twenty ten theme and changing it so that the layout, design and features match up with the previous design that I built. Should I go down the route of using a child theme to make the changes or should I change the actual theme, renaming it for the company?

I only plan on using the excellent ground work that twenty ten sets (compatibility/functionality wise), and fully intend on making sure its noted in the code that I used that theme as a framework, but is it ethical to do so?

I don’t do many websites as im only learning, and this would be the first word press site. I know its for public use and can be edited, but because im getting paid for my time im unsure where it falls.

Any help/advice would be greatly appreciated!

Cheers