After setting a watchpoint on MESSAGE, it stopped in the CHECK_EXISTENCE method of the CMD_EI_API_MAP_STRUCTURE class. It appears to be checking whether any of the variables it cares about are already in memory, then it specifically checks CENTRAL_DATA-TEXT-CURRENT_STATE IS INITIAL AND CENTRAL_DATA-TEXT-TEXTS IS NOT INITIAL and within the associated nested LOOP structure, it throws the error message I was receiving if the TEXT-TASK value was not M)odify and not D)delete.
I guess they are saying if CURRENT_STATE is not blank and anything is found in TEXTS, it should I)nsert the lines for the customer so M)odify and D)elete are not permitted.
So unfortunately, rather than setting CURRENT_STATE to blank to indicate nothing has changed, I now apparently need to set it to 'M' so it will skip this section of code.
Also, I created a local CMD_EI_API object variable and called the INITIALIZE method thinking it might have been related to my call to the MAINTAIN method, and although it did not change the outcome, I left it in there as a good practice.
I also needed to clear the tax jurisdiction code in case the changed address needed to redetermine it.
Thanks for suggesting the watchpoint!