I understand the "do it for all or for none" but honestly, that doesn't always work. And I resent outsiders (epspecially if it's not a regulation) telling us what we should and shouldn't do because of their own personal beliefs. I mean, I had a county employee tell me I charged too much for FT care. Should I have lowered my rates because she said so?
If questioned "their contract is different as they started at x time. I grandfather my clients in the contract they signed when they started. there will always be some differences between payment, avalible care dates, etc."
In my case new families always pay more than old clients, so I might point out that as well. Then I'd suggest that "I'll put you on the same contract but your rates would go up to x a week." I'm pretty sure I know what they'd chose.