The red jewels are pretty easy. StarLED or LEDtronics are two examples of companies that make drop-in replacements for most bulbs, both AC and DC supplied.
On a SSL 4K LED's work well on the meters because they point upwards, but in all of the switches they do not work well. I had strange connection problems, and some of the master section bulbs are IN the circuits.
I have completly LEDded the switches in older Neves, but not the meters. Even the mini-micro bulb in the gates.
With meters it is more difficult due to the LED's being directional. The UREI positioning would have the LED's shining at each other. I tried a breadboard of yellows across the top of a meter, but I didn't like the color. I have not tried any whites the same way. Same with dbx. The lights point into the front panel, so LED drop-ins would not work.