I have to add that just keeping a copy of the QT Player won't let you reverse to QT 6.3 so easily...
Qt is a set of extension build into the system to let you play all sort of multimedia files. The Quicktime Player is just a way to play those medias...
A lot of people thinks of Quicktime as the player without seeing the whole picture

The iceberg comparasion is the best to understand it
If you get imcompatibilties between 6.4 and some files, playing them with the old 6.3 player won't help you. You'll have to re-install the whole QT 6.3

(and if you're running Panther 10.3 - you just can't reverse to 6.3 as 6.4 is build into it)