Replacing all of the modules including the nodes would cost as much as building a new station. The specifications are limited, there's a limit to the mass of the modules that can be connected to the nodes (torque is quite real ;)) and considering we don't have the space shuttle to assemble new modules easily (with the exception of Zarya, Unity(node 1), and Zvezda all modules and large parts like the Solar Panel arrays were assembled with the use of the STO) so at best you'll end up with an ISS redux with the same limitations to size, mass, power generation and module compatibility as the current one has.

We need to be able to build bigger modules for bigger experiments, we need more power, we need it to be higher to further avoid atmospheric drag and to be used as a launch platform for further experiments.

There are various proposals for both low orbit space stations like the Russian OPSEK which will be recoverable, and various future uses for the ISS* including using Node 4 and some of the ISS modules for the EGP which will be a refueling station parked around E/M L1 (approx half way to the moon). The EGP could be build on the existing ISS and launched to L1 since it will have a central node and a service module additional modules even a habitat module could be later added.

* Please note that this will not reuse existing ISS modules in orbit but will reuse built/planned unlaunched parts like Node 4 and Zvezda 2.