I found three opensource projects for XML diff - xmlunit.net, Diffplex and xmlspecificationcompare.
But only MSXmlDiffPatch allows to patch XML-files. But it is under EULA.
How to patch XML file from CLI, using opensource C# program? Is there such program, which I didn't found? Or how to use programs mentioned above for patching?
RFC 5261 support will be good.
UPD: I found some library, which clams to be:
"tested with Appendix A
nearly complete, only 'Several Patches With Namespace Mangling' fails":
but it have no license.