In existing SharePoint 2007 applications you need to manually migrate the content of the default rich-text editor Web Part to Telerik RadEditor Web Part. Unfortunately, there is no supported way of doing that automatically.
For each page where you have a rich-text editor Web Part you have to perform the following:
Open the page for editing
Copy the content of a given rich-text editor Web Part
Delete this rich-text editor Web Part
Place a RadEditor Web Part on its place (as described in Using the RadEditor WebPart)
Paste to copied content inside the RadEditor Web Part and save it
Repeat steps b-e for all rich-text editor Web Parts on the page