I don't know exactly where is the Endorsement Policy stored.
May be someone from HLF TSC would help you here. But I know for sure that it is in no form which can be tampered with. But it is possible to change the endorsement policy of an already instantiated chaincode. Please refer to this link for a CLI based example Fabric Documentation v1.1 : section Upgrade & Invoke.
In addition to that, if I remember correctly, it can be done from the Java SDK as well. We used to specify a different endorsement policy yaml and change the version of the chaincode and used to install and instantiate. [ You need to change the version, so you have to install another copy even if there is no binary change :( ] As all the SDKs follow the same spec, NodeJS's upgrade API should also support this