Had a similar issue with a conversion from numitron filament displays to LED, where the resistor value selected, 82R, was making the display way too bright, likely because the original displays tried were old Monsanto ones, that needed 50mA to reach rated outout. The new ones were much better, but being loath to undo the cordwood board used as adaptor plate, and not wanting the pain of replacing 160 resistors as well, and soldering the 160 wires back to them, instead opted to use a diode in series with the common cathode of all the non multiplexed displays. Tried a single 1N5401, which made it dimmer, and a second one in series made a near perfect match with the original displays, though the diodes were running rather warm when you did a display test at full brightness, seeing as that made the current peak a little over 4A. 25mA per segment, at peak brightness, within the display capacity, as at 3 diodes they were too dull, and no room to install 6A diodes either, so 2 3A diodes, stuck to the mounting frame with lacing twine and conformal coat, would have to do. Bonus was that only had to do 4 units, getting all the non failed numitron displays as spares for the others.