There are many TT's that can be upgraded, it is a matter of how the upgrade is looked at.
Plinth Materials that are offering a improvement in comparison to the original selected material, where the improvement in stability of the alternative plinth material is much improved across different environments. It is quite common to see such items offered for sale.
A Tonearm or Cartridge is a simple and effective upgrade for a TT with these ancillaries that are limited in their function.
Improved Synchronised Motors are commonly seen available, with a function that offers a very tight tolerance for speed stability.
There are rethinks on Platter Spindle Bearings as well, where a different bearing assembly can be exchanged to replace a original design.
In some cases even the Platter Material is offered in an alternative material to the originals design.
Linn have excelled at offering their own Branded Add On Ancillaries, steering their customers away from considering the comparative devices on offer from competitors.
There are options to upgrade most TT's, the search for the method will be a little broader that selecting a sale item offered by a Brand to be used with their own Brand TT.