Lower voltage caps

I would say it really depends on application.  If you are using these for a speaker internal crossover network as a filter for midrange or tweeter, I would say that the 400V caps are likely going to be fine.  I have seen caps as low as 250V in a crossover network.

I think the one place you have to be very careful is if this cap is used as a compensation cap in a tube amplifier or preamp.  The power supply voltages for tubes get extremely high (sometimes as high as 385V-400V).  I would NOT substitute a 400V cap for a 600V in that particular application.

If the DC voltage across the cap at all times is below 400VDC, then yes. Otherwise no -- the cap will stress out and turn into a wire or an open in the circuit. And maybe burn and create a mess, ruining everything around it.

The 600vac will test out at 1000 vac it doesn't mean it will hold up at 1000 volts for any length of time..

Unless the unit came with 400 vac OEM caps, that's a no no, to go down from factory specs..

Regards