With WebWorks it is possible to run your application in both BlackBerry 6 and BlackBerry 10, you may need to make some changes to your code, but it is certainly possible and there are plenty of resources available.
You can start from here.
Specifically BlackBerry states:
Your BlackBerry WebWorks app can be targeted to run on a variety of
BlackBerry device models or ported to a different OS, such as iOS or
Android.
Developing your app with BlackBerry WebWorks gives you the flexibility
to target users of the latest BlackBerry devices and those who are
using older models. Depending on the OS you want to target, versions
of the BlackBerry WebWorks SDK available for:
BlackBerry 10
BlackBerry PlayBook
BlackBerry 7 and earlier
See here for a tutorial on creating an application which is able to work on old and new BB devices (Simple Hello World application).
A good article to look at can be found here, which explains some of the fundamentals and differences with webworks.