I have seen two different versions of this:

1. To build a shorter rod the line weight stays the same as long as you remove the butt section not the tip. It is simply a matter of purchasing say a 5 piece rod and building the top four pieces.

2. Even better yet purchase to identical 5 piece blanks say 9' 5wt and use the 4th section of the second blank to place a second handle & reel seat. This produces a 6 piece outfit that can be assembled full length or shorter version. Guide spacing gets tricky but can be accomplished - I have done it a few times. The trick would be to obtain the fourth section without the purchase price of the second blank.