First, understand that sand doesn't filter as finely as cartridges, so "upgrade" may be the wrong word. Cartridges filter down to about 10 microns, sand filters to 25-30 microns. (i.e.the cartridge will catch particles up to 3X smaller than sand will.)
If you still want to change, get the biggest sand filter available that you can comfortably afford. For superior results look at above ground pump/filter packages built by Hayward or Pentair. Intex products are just built to margin and are not designed for long term usage.
Answers & Comments
First, understand that sand doesn't filter as finely as cartridges, so "upgrade" may be the wrong word. Cartridges filter down to about 10 microns, sand filters to 25-30 microns. (i.e.the cartridge will catch particles up to 3X smaller than sand will.)
If you still want to change, get the biggest sand filter available that you can comfortably afford. For superior results look at above ground pump/filter packages built by Hayward or Pentair. Intex products are just built to margin and are not designed for long term usage.
Ditto to Bill's reply.
To answer your question... Don't get any Intex!
Look at Hayward, Pentair.