Two things to consider when applying TEVs:
1 - Capacity, which is based on the TEV port size & stroke, the thermodynamic properties of the refrigerant, and the conditions of the application. A TEV that is selected for R-12 will have approximately 20% more capacity when using R-134A...this due to the thermodynamic properties of R-134A.
2 - What thermostatic charge will provide the proper bulb pressure (applied to the top of the TEV diaphragm) to counteract the evaporator + spring pressure (applied to the bottom of the TEV diaphragm) such that the valve can maintain an operational superheat.
The characteristics of the R-12 thermostatic charge are close enough to what is required for R-134A to operate properly. In fact, a valve manufactured for R-134A is simply an R-12 valve, with the capacity rating adjusted for the new refrigerant. The factory R-12 thermostatic charge is identical to the factory R-134A charge.