You don't have to change the restrictors, you don't need the higher brightness because they'll shine very bright. I was thinking the same as you, nut then i decided to take lower brightness level and stay inside the limit of 1,9 volt of original. As long that you take same brightness level for same color you will not see a difference.The light intensity of an LED is actually specified at a certain current, not voltage. The higher the current flowing through an LED the higher the voltage drop across the LED. Since our bikes have 12V systems there are a number of LEDs in series as well a current limiting resistor and a transistor to turn them on.
Different color LEDs can have very different voltage drops. Generally, amber and red LEDs have the lowest voltage drop (often around 2V). The green, blue, and white LEDs usually have higher voltage drops (closer to 3-4V). So if the system voltage is 14V (when the bike is running) and there are 6 red LEDs in series with each dropping 2V, then you choose the value of the current limiting resistor so that you get the desired current through the string of LEDs. Having worked with similar LEDs before, I would assume that the stock amber LEDs are biased at about 25mA. Using LEDs with a higher voltage drop will require a smaller value resistor.
When I get around to changing these out in my gauges I'll trace out the circuit and share it with everyone. Unless of course someone has already done that and would like to post that info here?
Besides that it is a parallel circuit so voltage remains the same over all leds.