Can I put the new style A5 grip with selector switch on my older A-5?

Absolutely!  The A5 went through a design change awhile ago.  This is the newer grip with a selector switch safety.  This newer A5 is referred to as the 2011 A5.  So, Yes.   The A5 2011 Mechanical grip shown below is backwards compatible with the pre 2011 A5 markers.  This exact grip shown will also accept the response trigger kits but it will NOT accept the electronic boards.  This grip is the mechanical version and does not have the battery compartment built in.

Tippmann 2011 Mechanical Lower Grip Complete