Here are some instructions for PS3 to PS3.
Select the file from the PS3's XMB "Saved Data Utility". Don't do this in game, only in the XMB.
Once you find the correct file, use the triangle button to open the options to copt it to your USB stick.
Just copy it back to your new destination, be it a new user or different PS3.
If you want to copy to a new user you can skip the USB stick.
If you want to do it over the internet via computer follow these instructions.
1. Place USB drive into the USB slot on your PS3:
2. Go to the 'Saved Data Utility' folder in the XMB and open:
3. Once opened you will see all your saved replays and game saves:
4. Scroll down to find the desired replay:
5. Press the triangle button to reveal the replays properties and select 'Copy':
6. Select your USB drive device as the copy destination:
7. Copying... Please Wait:
8. Copy Completed.
9. Open USB drive while in your computer.
10. Right click the 'PS3' folder and go to 'Send To' and click 'Compressed (Zipped) Folder:
11. Zipped folder created and ready for transfer:
As long as you use a computer with a compatible zipping program you should have no problem transferring the file back to the PS3.