For the most part, no. If you change any of the drivers on an existing speaker, then you would need to rework, or build a new crossover, which is extremely difficult to do. It takes a very long time reading and studying speaker design to be able to build your own crossovers:
The speaker was designed and optimized as a complete system...woofer, tweeter, crossover, and enclosure. Making a change in one almost certainly requires a change somewhere else and those changes may not be simple or easy to figure out without measurement and testing gear. In many cases, you are probably better off getting a "better" speaker rather than trying to "fix" an existing one.
they can be replaced, they won't necessarily be an upgrade. Most likely would be worse. Look up your specific speakers and see if anyone has done modifications, sometimes people find really good replacements, or crossover modifications, or something to give good improvements.