To change the refrigerant in a Buick Rendezvous you will need to remove the old freon. Freon is bad for the ozone, so you will need to hook to a special machine and reclaim it. Next you will need to pull a vacuum on the ac syatem, then fill with new freon.
1993-95 for most.
A 1994 Buick Regal Custom uses 134a refrigerant, not freon.
R134
Where_do_you_add_freon_on_a_2000_buick_regal
R134a
R134a
R-134a
R134a
R134a
Some started in 1992 but most all started in 1993...........
You shouldn't have to change freon, it doesnt go bad.