>We changed our main program up to vfp8 (no problem with set enginebehavior 80).
>We have another program with hundred queries (many lines with group by 1), can I
>use with 'set enginebehavior 70' to this program - I don't ask about the reliability, I ask if it's can cause any damage to the data.

The SET ENGINEBEHAVIOR affects just SQL Select command which only reads data so it cannot do any damage to data.