EDIT: Yes, v20.06.00 unifies the two different minor revision paths of v20, you can upgrade any v20.x project to v20.06.00 and it will work just fine.

In this case, it is v20.06.00 if you were actually going to use v20.

You should be striving to use the latest minor revision of whatever major revision you use every time you do work on a project and this is true for every existing major revision of Logix/Studio 5000. v20 notoriously had two minor revision paths that were not compatible with each other, but Rockwell fixed that ages ago. Sidebar to address something glossed over in this thread. Prior to v21 the software you needed was called 'RSLogix 5000', but starting with v21 it is part of 'Studio 5000', but for purposes of programming Logix processors it's functionally the same thing (although they overhauled the interface somewhere around v30). Tl dr To open a v35 file or work with a v35 firmware controller, you need v35 software (same with any other version number). If you have a TechConnect contract you probably can download it from Rockwell using your existing license. In short, you need to install * specifically version 35* of Studio 5000. This Logix Designer is basically a repackaging of RSLogix 5000 (which was used for v20 and lower), and unless things have changed your license for RSLogix 5000 will allow you to use Studio 5000. 'Studio 5000' is the overarching name for the software package that contains the 'Logix Designer' which is used for software v21 and up. Is Studio 5000 necessary with this version 35 controller I was just informed that Studio 5000 was used to write the program.

Rslogix 5000 v20 free software download