You are replacing the source.
You are replacing the amp.
You are replacing volume controls.
You are replacing a 'few' speakers.

So in other words, you are completely replacing the whole system except for some of the speakers.

Why! Why would you not go ahead and replace the last few speakers and put a conventional system in?
If you say 'budget' I call BS. You need to educate the customer on why trying to save a few shitty speakers is not going to save them any money. And in fact, trying to Frankenstein another version of this same system is probably going to cost them more.
This just doesn't make any sense. Unless we are missing a lot more details. Like the system has 20 audio zones with 40+ speakers and you were only replacing 4-5 of the speakers. Even then I got to question the condition of 'all' the existing speakers, considering the typical product that would have been used for this and the age (neither of which you mentioned either).