Notify your customers well ahead of time that their current (Y type) account will no longer be available. I'd run it on a statement message for at least 2 cycles and also send out a mailer probably. Give them a date, in writing, by which their account must be changed (by them) to some other type of account (offer them some alternatives and the basic info with a "see a personal banker for more details" disclaimer). In writing also provide them the date on which (if they do nothing) their account will automatically be changed from Y type account to X type of account. Provide all disclosures that apply to X type of account.
"Gratitude makes sense of our past, brings peace for today, and creates a vision for tomorrow." - Melody Beattie