In the Webdriver documentation, they explain a method to start migrating from Selenium RC to Selenium WebDriver.
Basically, is creating the selenium object like this:
WebDriver driver = new FirefoxDriver();
Selenium selenium = new WebDriverBackedSelenium(driver, "http://www.yoursite.com");
the main problem with this migration (instead of changing the whole code) is the wait for page to load. As they say, the command WaitForPageToLoad returns too soon. The getEval is another command that you have to change.
I think that the best approach is to make functions with the main commands that differs from Selenium RC to Selenium WebDriver, and, once everything is "working" keep modifying your code until no Selenium RC is present. This is how we did the migration, and we had lots of lines of code.
This is the link, where they explain how to start:
http://www.seleniumhq.org/docs/appendix_migrating_from_rc_to_webdriver.jsp#migrating-to-webdriver-reference