I don't know the 400 series, but the 345 and the 375 have 030 daughter
boards and can indeed be upgraded.  You need the 040, a new oscillator and
in my case, new boot ROMS.  The old oscillator was on the daughter board
and the new one plugs in right next to the 040 socket.  The new oscillator
whould be twice the frequency you want to run the 040.  My old ROMS didn't
pass self tests with the 040.  It took a while but I tracked down the right
ROM parts for the 375/345 from HP, and I can give you that info if you want
it.  No idea about the 400 series, but I'd venture that it's worth a try.

Dave

> In the HP Series 400s:
> 
> Is it possible to replace the '030 daughter board with an '040?
> Does this require anything else beyond the '040 processor?
> 
> Ron