While you can spend $20.00 per lamp to convert, you don't have to. If you are willing to wait for the long delivery times, there are many types of LED conversion lamps available directly from China through eBay for a few dollars per fixture. At that price, they can be treated like a standard incandescent lamp - if it fails, toss it.
A couple of points when ordering these eBay lamps - Decide ahead of time what color temperature you want. Warm white 3000°K - 3200°K are closest to incandescent lamps, however the cooler 5000°K lamps will give more light
for the same power input. I'd avoid anything cooler than 5000°K (cooler is a higher color temperature - don't ask why) since the output will be too blue for practical use.
Another variable is light output. It should be measured in lumens - the number of SMDs (the individual light emitting chips) that make up the lamp doesn't matter since they come in different outputs, so overall lumens is the best way to compare.
Make sure the adapters sent with flat panel lamps match your sockets. If not, it may be difficult to find an adapter (although it is possible to make them from old incandescent lamps).
LEDs are polarized, that is they will only work when connected + to + and - to -. Since incandescent lamps don't care about polarity, not all sockets are wired correctly - if a LED replacement doesn't light, try switching the leads to the socket.
While it is tempting to purchase one or two LED lamps to see whether they are the right color, brightness, etc, the problem is the next time you order the seller may have a different batch that won't match, or even be out of business. My suggestion is to order enough to do a full replacement plus a couple of spares. You will often get a better price or free shipping with orders over 10 units.
Lastly, many of the inexpensive LED replacement lamps do not have a voltage regulator built into them. This is good & bad - the good is many voltage regulators will produce interference with radios & TVs. The bad is that if your converter goes into the equalization stage, the 15v+ may damage a 12V LED. I have not had this problem, but others have mentioned it. The way to tell from the lamp description is those with voltage regulation will show a voltage rating as a range, typically 10 - 30V while the non regulated lamps will be labeled as 12V lamps.