Change a 2 channel amp to a mono block amp

Agree the amp must be designed to accept switch to monoblock from the start.
And all I know of which are, also mention that fact on advertising. No amp companies keep it a secret.
So unless the amp company states it CAN be used bridged.. You have to accept that the amp cannot be bridged.

It might be possible (if each output isn't already in bridge configuration) by shorting grounds together and connecting speaker between outputs. Input to one of the amp has to be inverted. This will get you 800W but as Mofimadness suggested it is not wise to do, since amp was not designed for that (power supply, heatsink, output transistors etc).

Do the speakers have bi-wire option? If they do you can bi-amp them! Otherwise like Mofi said,forget it!

Unless the stereo amp is configured internally for mono operation, you can NOT bridge it. Don't even try, flash...puff...smoke and maybe a tripped circuit breaker.

20 or 30 years ago, there was a company that made a little box that supposedly would bridge a stereo amp, but it didn't work very well.

If you do a search on here, this has been discussed several times before.