For those of you who don't know what we mean by the words 'taper' as it applies to a bamboo rod - we mean the diameter of the rod along its length. Obviously, it will be thicker towards the butt end than at the tip. It's how the rod's diameter changes as you move up (or down) it that will dictate how the rod acts & feels. There are many ways to say this (some talk about mass of the rod, deflection points, etc), but to me the best way is to say that a rod's taper is the combination of it's diameter at a set of equal points along the rod's length.
So the taper is, essentially, the rod. Different tapers do different things & the combinations of tapers are endless. That's why there's so many different bamboo rods out there. Naturally, then, these tapers can be adjusted to slightly change the characteristics of the rod. It takes a lot of work & thought on the rod maker's part to do this, too. A lot of contemplation has to be done before you begin splitting cane, else you run the risk of making a rod unlike what you wanted.
This usually isn't a problem because there are a lot of tapers out there. It may take a little time & digging around, but a rod maker can usually find a taper that will do what a client wants their rod to do. Slight adjustments might have to be made, but otherwise this is the best way to go because there you're sticking to something that's been proven successful over time.
Often, someone might want a particular taper, but want the rod to be made for a different line weight than the taper they're after. In other words, they might love the feel of a Payne 98, for example, but rather than a 4 wt (as it was originally designed for) they'd prefer it for a 5 wt. Can this be done? Usually the answer is yes. A consistent change to the numbers, or diameters, in the rod's taper can allow you to do this.
Where we get into trouble is when we adjust the rod taper too much. True, the diameters in a taper have a relationship with one another, but when you start making a rod a lot smaller or larger than it was originally designed for you begin to run into problems where the rod will not act in a predictable way. In theory it should work, but my experience tells me that when you start jumping a taper 3 or 4 line weights, bad things can happen. The rod you get is nothing like what you expected.
There are many reasons for this & the biggest, I think, is the bamboo itself. If you're making bamboo strips that are very large, the material becomes hard to handle - i.e. it will fight you & won't wont to cooperate with you. It's very difficult, for example, to heat straighten a very thick piece of bamboo. That's why you see that some of the largest bamboo salmon rods are 'double-built', meaning there's a smaller rod inside that larger outer one. When you get down to the smallest of rods you begin to have a rod that's not really practical to fish with.
So changes can be made to a taper, but we want to keep it within reasonable limits. Figure a decent rod maker can make a rod to within tolerances of say .002" to .010" or there-abouts. Stretching a taper too far out of it's original size can make it even harder to stay within tolerances & it becomes an unpredictable project. It's always better to get as close to what you want with a proven taper.