1. Is this an Administrative Modificiation (unilateral)?
2. Who can sign the Modification (what threshold)?
3. Is the Warrant threshold value based upon value that you are adding to a contract or that you are obligating?
Yes, the scenario you described appears to require a unilateral contract modification. According to FAR 43.103
, unilateral modifications are used to make administrative changes.
We recommendethat you review the contracting officer’s warrant authority and your agency’s supplement and/or policy as well as seek additional guidance from your legal office regarding the authority to sign the modification. The FAR nor the DFARS indicates the limitation of a contracting officer's warrant as per action or cumulative/aggregate.
You are advised to review the specific wording on the contracting officer's warrant. Generally most limited warrants are tied to obligation authority for specific actions. The direct answer to your question depends on the wording in the contracting officer’s warrant authorization.