The chart of accounts structure and structure instance are fundamental constructs in General Ledger accounting setup and cannot be altered once they're used.
The number of segments, the segments' order, each segment's label, length, type, and value set assignment cannot be updated.

These components set the foundation upon which accounting data is recorded for ledgers that use them.

If you have to, then it will require creation of new Set of Books, Accounting structure, Multi org and set up all modules again and perform conversion from old COA to new.