PO Charge Account Update, Future Segment Changes

Hello, We have changes to existing accounting segment which meant to be used in future. So now we are implementing changes on future segment.

In the process, how to update accounting on existing open PO lines which are billed partially. Is it right to update accounting on partial billed line directly?

1) what is the right way to update for new segment on the PO charge account?

2) Do we have API to update or any automated process to run which will update and add new line with new segment for PO charge accounting changes?

Thanks in Advance....