Do all LEDs have the same optical power vs current curve?
LEDs are unlike lightbulbs where you apply a voltage and then a current is drawn (depending on the bulb's rating). LEDs have to be provided with a certain current (indicator LEDs usually need 20 mA or less) otherwise they'd draw too much current and destroy themselves.
So a series resistor needs to be used. That resistor then sets the current.
If you replace an old (indicator) LED with a modern one, indeed it might be much brighter. The safest and simplest way to reduce that brightness is to increase the value of the series resistor. Since a higher value is needed, you can simply leave the existing resistor in place and add a 2nd series resistor. You can mount that close to the LED if convenient. Start with a 1 kohm resistor and see what that does.
The first part of your question can only be answered if we have datasheets for all LEDs. But I think investigating curves etc. is overkill as you simply want to replace some LEDs and make them burn less bright. And that is easy to do.