Why are U replacing the 12SEER condenser?

You should perform a heat-gain heat-loss load-calc to determine if U need a 3-Ton or smaller condenser; when properly installed, smaller is much more efficient & dehumidifies much better.

Click on the load-calc link:

Here is a link that might be useful: Wholehouse load-calc

Coils by themselves don't have a SEER rating. The SEER rating is determined by the combination of a condenser, coil, and air handler or furnace. The efficiency of the three pieces of equipment are rated as a system. If you change on of the components, it is likely the SEER rating will change.

If your current condenser uses R22 refrigerant then you will have to get a new coil if you switch to the R410 refrigerant. The two refrigerants cannot be mixed together. It may be possible, but it is not practical to flush out an older coil. It is best to replace it.

There is a directory, the AHRI directory which lists the system efficiencies for tested components. I am sure the ADP ( owned by Lennox) coil is listed with your other components. Mike's advise is spot on here. I can't say that your evap. Is one of them, but it seems the industry has been suffering from pre-mature evaporator coil failures.

As to the 22vs410a. The 410a is the better refrigerant. More friendly environmentally, more efficient, less expensive going forward. Your line set has to be either replaced if practical or thoroughly flushed before the 410a can be used. 22 residues screw up the chemistry of the refrigerant. Don't let the old crap compromise the new equipment.

Supposedly R410A is more environmentally friendly, but other than that it is not a "better" refrigerant, either for the consumer or the contractor. In fact, it can be worse in some cases, R410A systems operate at higher pressures and can be more sensitive to their installation conditions. i.e. they often can't survive hack jobs as well as R22 systems could.