1) As you can see from the other answers, there is no simple answer - a few units out there can simply have pressure regulators changed, be flushed out, have your new condenser unit installed, and be charged with the new gases. Others require new evaporators and/or tubing because of the higher pressure needed for R410A, and in many cases the compressor has to be replaced too - so pretty near the whoope system except for any ducting may haveto be replaced, depending on your make and model. R410A is actually a mixture of two other refrigerant gases - R32 and R125, and some units have been found to corrode in contact with one or the other of these gases.
2) There are also alternative gases - R44 and R424A that can be used instead of R22 and, in SOME units, can be run without modification other than flushing the system before putting in the new gas.
3) Therefore, you are trapped by two factors - first that R22 is being phased out and getting more expensive every year, and while it will be available as long as your current unit is likely to live, if you have a leak in the future it may end up costing $1000 or more to refill it. Secondly, since the condenser unit which has to be replaced is pretty pricey already, you need to weigh the condenser replacement and possible future costs (including compressor life) against the cost of installing a new unit now. Obviously, whether your current indoor evaporator/fan units can handle R410A or would need replacing would be a significant consideration in your decision.
4) If your unit is less than about 10 years old, you have a tough decision, and you need to factor in your financial situation and how long you plan on living in this house - certainly a minimal-cost repair now makes more sense if you move every few years, whereas ifthis is your forever home and you job situatiuon is not likely to force a move, then a new unit might be less hassle and make more economic sense. Ifmore than 10 years old, then I would personally say look at a total replacement now, especially if your unit bled off the Freon so you are goingto be paying for gas replacement cost of probably $500 or so as well during a condenser replacement.