Can line number change for a contract line?

We are creating contracts and lines via the contract import interace.  The requirements say to use the line number from a third party system to create the line in OKC_K_LINES, then another process extracts anything rebooked to send back to the third-party system using the original line number as the key.,  I argue since line number in OKC_K_LINES is not party of a key to the table, we cannot use it as a key to make updates back to the original syem.  So I wonder if anyone has some suggestions on what steps I can perform to show how a