This response is based on the information provided.  We suggest you discuss with your contracting team, program manager and/or legal department as appropriate.

This is not really a contracting question.  Key to your question is, "What type of money is on contract and for what purpose was it intended?"  If the money (whether O&M or R&D funds) was originally applicable for use for labor or ODC purposes then there is nothing that prohibits it from being moved from one CLIN to the other.  One caveat is that if the funds in question are O&M funds and the useful period has expired (Sep 30 of that fiscal year authorized), the funds are not to be de-obligated from one CLIN and obligated to another.

The severable/non-severable issue relates more to the fiscal year(s) of funding when awarding a contract crossing fiscal years goes to bona fide need.