ORIGINAL: Raz
Safest way would be to replace the antenna, since a bad solder could cause resistance [:-]
Replacing the whole antenna also requires soldering! You have to un-solder the old antenna and then solder the new antenna wire to the receiver circuit board. The receiver circuit board is small and rather delicate, and you have to remove it from the receiver case in order to replace the antenna. Not only that, but you need the correct tools and some experience to work on small circuit boards.
Soldering the 9" piece back onto the antenna is far easier and less likely to cause problems.
Another option is to leave off the 9" piece and use the shorter antenna as is. I don't recommend this, but it will probably work if you don't fly your plane too far away from you. If you do this, be sure to carefully check the range of the receiver with the short antenna before you try to fly.
- Jeff