Eclipse variables such as openstm32_compiler_path can be changed by opening the property dialog (Project -> Properties) then selecting C/C++ Build -> Build Variables
Make sure to check the “Show System Variables” option on the bottom of the dialog.
I tried editing the variable mentioned above (which was marked as a “ECLIPSE DYNAMIC VARIABLE”) to point to a different location for the ARM GCC tools, the override was accepted, and shows in the Build Variables dialog with the changed value, but it did not appear to “take”; the AC6-bundled tools were still used.
It is possible that you might have to edit the PATH (same dialog) since the path to the AC6-provided toolchain appears here as well. I did not take my own experimentation that far.