Odoo Ent. Ed. v15

I am trying to add an analytic account to an existing Sales Order but that field is disabled.  The Analytic Account setting is checked in Settings, I am shown as an 'Accountant' and I have administrative rights.

Although I may be mistaken, it appears that I confirmed a quotation without an analytic account and then, from there, created a sales order (also without an analytic account) and, from there, created an invoice with a 50% down payment.  The quotation now shows the 50% down payment as a line item, the Sales Order, likewise shows a 50% down payment line item and there is now a 50% confirmed invoice (In Payment).  I also show a draft invoice for the same Sales Order number that deducts the previous 50% down payment amount and will invoice for the balance.

The Sales Order has been given to a customer so I'm hesitant to delete it and to create a new one.  I'm not sure why the analytic account field is disabled but, in any case, can someone suggest a solution that does not require editing Python code?  If an analytic account now can't be created, for whatever reason, that would be helpful to know as well...I'll just have to recreate things and do some explanatory documentation.