Converting from R-12 to R-132A is fairly easy. There are two schools of thought. School one is simply replace the gas. The procedure it this, Using a freon sucker device totaly evaucate the system, Remove all the freon (this requires professional grade hardware) as it's valuable, very valuable, and sell it to a recycler.
Seal any leaks in the system (Most important) and do any other service needed to the system. Purge it, Purge it well
Finally again evaucate the system and then refill with R-134A
The other school of thought says you need to replace all the hoses and seals (Basically anything NOT made of metal.
Note: this is not as expensive as it sounds by the way, there are not that many hoses (4 on most systems, sometimes just 3)
I would suggest contacting a professional ans asking about the process
Now, the bad news
R-12 systems will drop the temperture 30 degrees in most cases, so air entering the evaprator at oh, say 80 degrees, comes out around 50.
R-135a only drops 20 degrees for the same system, so you loose a LOT of cooling power