Can I change an orifice to make an R22 evaporator unit compatible with a 410a unit?
The orifice will need to be changed. The question is whether the evaporator is rated for the 410 pressures. In the early days the R22 evaporators could not contain R410, but after 410 came out mfg’s started making heavier coils that could support 410 but R22 was cheaper and we had experience with it so we continued using it even though it was being phased out. 

If the evaporator coils are rated for R410 sure go ahead and use it, but as I told another person on this forum today I know of a company that upgraded an old R22 system it worked until a hot day and the evaporator coil exploded causing damage to the air handler and the home having to be evacuated (Freon is heavier than air and can kill especially with a closed up house). The fire department aired out the house but the oil in the air handler, furnace and ducts had to be removed before the home could be occupied because of the smell.

So I would check the model of the evaporator to make sure it has an acceptable pressure rating. If so, change to the appropriate size orifice. This won’t be as efficient of a system as it could be but it can work if the pressure rating is there.