Can I edit a template after it's already been in use?

The short answer is "it depends".

There’s two parts to every template; the editable part, and the non-editable part.

Any changes to the editable part will not affect existing jobs.
Any changes to a non-editable part will affect existing jobs and output from existing jobs will reflect the changes.

Some more things to keep in mind:
Changes in style definitions can have weird effects and might cause the display of text in the web editor to differ than the output from InDesign Server. Changing style definitions in existing templates should be avoided.

When editing templates the following characteristics of the template MUST stay the same, or there will be errors with existing jobs
1) The number of pages
2) The page sizes
3) The measurement units used in the document