First, the PL400 VU meters are occasionally available, the PL700B meters are virtually impossible to find; those would have to be aftermarket. Then, you will need to find replacement Faceplates for your amplifiers because the meter dimensions are not the same. The replacements would have to include the VU meter bezels and other attaching hardware. Finally, the meter lighting would need to be reduced from 12VAC to 6VAC.
I am not sure this has ever been done. You may have more success purchasing a PL400 or PL700B and upgrading them to perform like the PL400 Series II and the PL700 Series II. Much easier.