A lot of stories have been written about how one guy, or another, modifies a certain lure in order to get it to do what you want. I know one guy who spends hours weighting Suicks and testing them in his bathtub before he takes it out fishing.
How much will modifying a lure help your odds at catching fish?
There are some lures that are just better at catching fish, like Rapalas, some that work well, but don't seem to have that fish catching magic, like Depthraiders and Shallowraiders, and there are some lures that do not work well at all.
If you buy a lure with the fish catching magic, you'll only make it worse by changing it.
If you buy a lure that is good already then maybe you'll improve it, or maybe you won't.
In either case you'll be spending fishing time playing with your lure rather than fishing.
If you have a specific case where you want something that doesn't exist, then by all means make it or improve something until you have what you want. You will miss fishing time, however, because you will be trying to get your new lure to work "just right."
Unless you want to spend hours every time you want to modify your
favorite lure, I suggest using lures that work well right out of the
box. Or those that only need minor modifications.
Some minor modifications you might want to make are: switching saltwater hooks for bronze hooks (bronze hooks disolve in fish stainless do not), adding hooks, subtracting hooks that get tangled often.