I dont think it would be wise to change just one for a smaller vac, best to do both at the same time,
The fitting is easy, just need a few bits of plumbing accessories form b&q and your away.
The flanges are a bit smaller on the 5.7 vacs so you have to step them down a bit but easy enough.
Ive not noticed any difference in power with them and the machine is being used nearly every day.
I got cheesed of with replacing the so called 'super' vacs, at one point i had one that lasted 3 months, and no i it wasnt left in the cold / damp in my van or garage, but in my home, and it didnt have foam go through it.
I think the last one i bought from extracta was about £250 which is just ridiculous, i could have five of the smaller ones for that
Even if the only last a year its still going to work out cheaper, I'm sure the bigger ones just get too hot or something and destroy themselves or are faulty in some way, I dont think i ever had one last longer than a year.
The one and only time i sent the machine for a service to them it came back really damaged and no one would take responsibility for it so I vowed never to do that again.
To be honest there fairly simple to work on, and as a bonus with the smaller vacs theres more room to move around inside.
Not saying this is the only or best way to go but just what my experience has been, its just when you start chucking money down the drain you start to think or cheaper ways of going about it,
Regards
Steve