Yes, the choice is yours, but there's also the question of whether there's any practical reason to make changes, and what kind of person you are. Didn't imagine there might be a moral issue here, did you?

Just be logical. Take into account what's working, what's not, and what it will cost the client to make the changes. That is, unless you're one of those sales pukes who walks into a house, waves his hand, and says "all this shite's gotta be replaced!" I'm hoping you're not one of those.

First, I'm just assuming the rather bulletproof SMS-10 is working!

If the VCs are working properly, there's no reason to replace anything.

If one VC is dead or malfunctioning, the cost of replacing one VC is less than the cost of four, so replace the one. There's no reason to change to 4 IM VCs and ditch the SMS-10. (Pretty much all VCs are IM type now, so there's no issue there that would cloud things.)

Two bad VCs? Three bad VCs? Now you're in decision land. Do what's best for the client.