We have been charging a particular type of account, a NOW account, a service charge of $12. We have decided to drop the service charge to $10 for all accounts opened from this point forward. Is there a TISA issue for not dropping the fee from $12 to $10 for the older existing accounts?

No, there is no TISA issue. However, you'll need to be able to distinguish between "old" and "new" accounts when applying your service charge routine, and don't make the mistake of handing an "old" customer a disclosure for your "new" account if he asks for information on the "old" account.