Can I use the existing evaporator or would I need to replace the evaporator coil


How about using a 4 ton 10 seer unit

You can keep the existing a-coil if you size the orafice correctly, or have a txv valve. However, it is best practice to have the a-coil and condernser matched. I would also suggest having a minimum 13 seer installed(coil and condenser), as ten seers is a thing of the past.

Just curious why are you up-sizing the outdoor unit? I also agree you should jump to a 13 SEER unit.

I`m guessing you are replacing the 3 ton with a used 3.5 ton condenser,, around my state there are no more new 10 seers units to be had
If this is the case you won`t have a warranty to worry about,because the condenser and evap coil don`t match