FritzF, the message received is "You cannot change the level of an account that is associated with a 1099 category. To change the level of this account, you must first delete its association in the 1099 Preferences." I checked the 1099 mapping list under the Preferences/Tax: 1099/Company Preferences (requires single-user mode) and by selecting "click here" following the words " Form 1099-MISC".
This opened a new window with a table "Accounts used for 1099". The account in question was not listed.
After some further research and more attempts to change the sub-account (without success) I came back to this window. Out of curiosity, I changed the selection in the pull-down from "Show 1099 accounts" to "Show all accounts" and - surprise - the account in question was listed. I changed the setting for the account to "Omit these payments from 1099" and was able to change the sub-account.
Problem solved, after three hours of attempts, research, and talking with our resident QuickBooks experts.
