I am a little nervous after reading an earlier post regarding the new stuff attacking the hoses.
It's more a case of the new refrigerant being able to seep through the older hose material than actually "attacking" it. Hoses intended for use with R-134a are referred to as "nylon barrier" type, which is a fairly self-descriptive.
Those Wal-Mart retrofit kits are a crapshoot - not only are the newer hoses different, the o-rings in the system fittings are made of a different material, too. Compressor crankshaft seals, too. Changing the service fittings and adding new oil (with "conditioner") and refrigerant might work okay, or your compressor might lock up after a week or two.
The "old" oil in the system will turn into sludge when exposed to R-134a, so most A/C technicians recommend a thorough system flush (not just pulling a vacuum) to remove as much oil as possible, before adding the new stuff.
R-134a can only be charged to 80% of the system's R-12 rating. That means you're starting out with 20% less cooling capacity - and a larger, more efficient condensor might be needed to keep the cooling capacity from dropping further.