You want to change the account assignment category in service documents (EBAN-KNTTP, EKPO-KNTTP) but the field is grey or (using BAPI) you receive information message ME 664 "Change Acct Assgt Cat. could not be effected". The current account assignment category is not U (Unknown).
As per standard system design it is not possible to change the account assignment category for service items with account assignment category other than 'U'.
It was designed like that because the accounting is made at SERVICE level and not at ITEM level, as for materials. For one item there can be several services with different account assignments.
Therefore a change of the account assignment category at item level is not possible.
You may also check note 360632, ad point 6:
"It is now possible to change the account assignment category from 'Unknown' to something else. Not however from (for example) 'cost center' to something else. This restriction only applies to service and limit items. The field selection must also be set as a prerequisite as described in note 313120 ."
Related notes / content:
360632 ME21N Various improvements for account assignments
313120 ME21N:Accnt assgmt cat fr item cat D cannt be chngd