We do conversons all the time and frankly the sticker is a joke. If the car has the new fittings its safe to assume that it has 134 in it.
Why would someone charge the system with R12 and then put the new fittings on?
If the system isn't working and you jump the low pressure switch it seems as if the charge is low. You said it was just done so I would say its time to get it to a shop that has a leak detector and find the problem.