>If we upgrade to VFP 9.0, do we lose the capability of compiling for 6.0?
>We anticipitate needing to compile at both levels.

No. As long as you don't use any of the newer extensions and improvements, like SET ENGINEBEHAVIOR, icons in menus, correlated queries or any of the expanded queries possible in VFP9, new report engine, BLOB fields, etc. you could always compile the same source back in VFP6.

You could also put pre-compiler directives around version-specific code. Something like:

#IF Version(5) > 600
  * newer code
#ELSE
  * older code
#ENDIF