If you are replacing existing incandescent 12V bulbs you will need to replace the transformer with a.
Do led bulbs need a transformer. Much more in fact. It seems fickle with a lot to be ironed out still. I was under the impression LEDs damaged transformers or the bulbs and they needed drivers.
In other cases you need to replace. As explained above the transformer you require to do this depends on the combined wattage of the bulbs youre powering. If the LED bulb requires DC voltage input it cannot be used with an AC LED driver.
Mains voltage LEDs such as GU10 and B22 bulbs have the driver built in to the bulb. Some LED bulbs like the Philips Master LED range have in-built circuitry that can deal with most but not all transformers so you dont have to change them. If you are replacing existing incandescent 12V bulbs you will need to replace the transformer with a special LED driver.
We also sell 12V light bulbs but these need to be used in conjunction with a 12V transformer. If you are replacing existing incandescent 12V bulbs you will need to replace the transformer with a special LED driver. MR16 and other 12V bulbs use an external driver.
And so there is. Sometimes you have to replace the old transformer with an LED driver to control the voltage and stop it blowing the LED especially if you get lots of power surges from thunderstorms for example. All mains powered LED bulbs require a transformer.
For the simple reason the GU10 LED bulb has an internal driver. And the bulbs are getting brighter and more energy efficient every year. The LED light bulbs we sell operate on a UK standard 240V AC and the light bulbs have a built-in transformer to change this 240V to the LED required voltage.

