Art,
I tend to follow a web site
www.griffiths.com.
They state:
G) The original rubber a/c hose lines in your car will leak out R12 through the hose's lining. R134a has a smaller molecule than R12 so R134a will leak out sooner than R12. So if you car has a large amount of rubber hose, such as a 40 feet with the 911, then you will be re-charging the system often and that is expensive no matter which refrigerant you use. To prevent refrigerant loss you should consider replacing your old hose lines with "barrier" type hose.
At least change your "O" rings to R134 compliant.
You will read here that many do not change the hoses. R134a is cheap to refill so why go to the bother?
John_AZ
1988 924S R-12
1987 924S R-12
1998 Cherokee R134a and topped up with HarborFreight AC hose + guage=EASY