I am looking for guidance on what level is required to do that? What DOD or DODI documents can help me? Also, for KPPs, KSAs, and APAs, who approves that?
Generally, the PM must achieve the threshold (T) requirements of all the KPPs exactly as written in the CDD unless relief of those KPPs is provided by the JCIDS process (see discussion below on AROC or JROC memorandums), or as a result of a Configuration Steering Board (CSB.) CPDs are still valid and have the same rules as CDDs. The PM must next work to meet KSA Thresholds and then APA Threshold requirements as the cost and schedule support meeting those threshold requirements.
To your specific question, if the transportability attribute is not a KPP, then the PM may make it part of the program strategy to not meet the full requirement because of a number of valid reasons. The PM should work with the Requirements Manager/Sponsor to determine the ramifications of waiving these requirements.
CJCSI 5123.01 covers JCIDS along with the JCIDS Manual (August 2018 is current as of today.) DODI 5000.02 covers the Adaptive Acquisition Framework along with a series of DODIs 5000.XX for each of 6 acquisition pathways.
You will find guidance on changing CDD requirements within each CDD's Army Requirements Oversight Council (AROC) Memorandum that validates the CDD or if joint, the Joint Requirements Oversight Council (JROC) Memorandum. In the CDD's validation memo (whether AROCM or JROCM) the validation authority normally issues guidance on changing requirements. Normally, they defer to the sponsor, i.e. the senior commander or executive of the document's sponsor, to approve all other changes that are not specifically withheld by the validation authority. And those that are withheld are normally limited to the KPPs.
The sponsor, therefore, may adjust all other requirements not withheld by the validation authority. A memorandum should be written and signed by the commander or senior executive of the sponsoring warfighting function documenting the change. The memorandum is then attached to the original CDD in CAMS and KM/DS by the Army Gatekeeper.