How do convert r12 freon to r134a?

There may be a difference in how each gas effects different seals & gaskets, so they probably should be changed to compatible items. But the cost may not be worth changing until there is a problem if any, then get a recon done.

On a vehicle or house system, pump out the Freon r12 into a gas bottle (well vacuum the bottle 1st really to suck it in).

Then pump in R134a (well vacuum the aircon system & then it sucks in the R134a).

Or - just ask a Refridgeration maintenance guy to filler-up & check for leaks.