Hi,
Perhaps you come from an Apple FCP background, where source clips were QuickTime and exporting also to QuickTime, and apparently these clips could sometimes be stitched together (I'm a PC editor).
Aside from a few formats that might offer some kind of "Smart-Render" under the hood, you will ALWAYS need to render any export even if the source and destination format seem the same or similar. Making a small change and exporting again does require a re-render, this is true.
Now, if you had rendered parts of the timeline (due to heavy effects applied) to green status, and then in Export you check the box for "Use Previews" this can reduce render time. Those "green" Preview clips are still getting rendered again to the final format choice, however the effects do not need to re-render and that can reduce render time of course. Depending on the sequence Preview Render format though, you may not want to use the preview files as it might reduce final quality.
If you can post a screen shot of the Export Settings panel, it may help to find a way to streamline your workflow.
Thanks
Jeff Pulera
Safe Harbor Computers