SAPGUI backward compatibility document

Dear all,

Where can I find some document stating that SAPGUI versions are backward compatible? We have SAPGUI 6.40 now. Applications are from 4.6B to ECC 6.0. We want to make sure that in the future upgrade of SAPGUI, we can still access legacy 4.6B, 4.6C, 4.7 systems.

Thanks.

Joe