How to get the best controls upgrades?

We do a lot of controls upgrades including panels with a lot of old mechanical relays. Our first response is to simply replace them all via PLCs. Some of the non-variable functions, though, are ideal for relays, and many of the installations are in high-EMI areas where relays do better than PLCs. Customer reaction is mixed. Some want every possible relay eliminated. Others who have had them a long time don't see the need to ban them entirely. Anyone dealt with something similar?