Ive been working with Infopath 2007 and my own XML Schema recently. After I published the form to a MOSS2007 document library I found that all the fields and groups in the data source were now locked and had a padlock against them. Even if I tried to edit the local copy of the XSN I couldnt change the settings.
Ive read that InfoPath locks the schema for imported and connected data sources, however this is my own schema and its locked me out of it.
I urgently needed to add a field so I saved the Infopath form as its constiutent files and edited the XSD, Template.XSL and the Manifest XML files manually. The opening the manifest file into InfoPath brought in the additional field (still locked!!!), but usable.
Ive read that you can use Tools -> Change Main Data Source for this however InfoPaths heavy handedness in the respect of a local XSD schema is rather worrying.