Put your precious VHS tape to one side - take two Cassettes that you can spare and a small cross-point screwdriver. Set them on a table and practise taking them apart and swapping the tapes between boxes. When you're confident that you can do this and still make the tapes work properly - take apart your precious tape box and move the reels and tape to a new box - it should work fine if you do it carefully.
Good luck...
Peccavi, February 2010