Can I upgrade my Single Site License (SSL) to a Developer License (DL) at a later date?
Yes. From your order history page click on the "Upgrade" link next to the product you wish to upgrade. The upgrade price is simply the difference between the DL list price and the original SSL price you paid. For example, if you purchased the CMS Suite v3 for $195 and then wanted to upgrade to the DL (regularly priced at $495), then the price to upgrade would be $300.
*You do not need to update any files in your existing installation.