I have tested this on my IC-821H and it worked, needless to say I assume no responsibility for damage to your rig.
D101 thru D108 are the relevant parts (diodes) and are located on the display board. You will have to remove both up and down covers and flip-down the front panel to gain access to the display board. 'Installing options' section of the user manual describes this procedure.
D101 thru D108 are all SMD parts mounted near the center of the display board forming a single row, D101 being the one closest to the S-meter. Note that some diodes may be missing on the row, in my USA-version
of IC-821 D101, D102, D106 and D108 were missing.
Removing D101 extends VHF RX (to 136-174MHz) (default for USA ver.)
Removing D103 extends UHF RX (to 420-460MHz)
Removing D105 extends VHF TX (to 136-174MHz)
Removing D107 extends UHF TX (to 420-460MHz)
There is no need for resetting to make this mods effective.