What would be the process to extend the schema during migration or to change domain (forest) mode? Is this recommended?
This question is purely process related. Support recommends contacting your Account Manager and arranging discussion about migration best practices with our architects and Professional Services engineers. Generally it is not recommended to modify or change anything in your environment while in the middle of migration.
Support did have cases where during migration customer decided to upgrade to 2003 and extended schema (similar problem can also occur when changing domain mode from mixed to native 2003).This introduced the new attribute (inetOrgPerson) damaging the directory sync.As a result domain pair had to be deleted and recreated again with the requirement of doing Full Resync (remigrating all objects). Please read more in the following KB article:
Synchronization is broken after schema on the source domain was upgraded to 2003.
User Guide, part Pre-migration Activities says:
Before you start directory migration, analyze the existing directory. This includes identifying required hardware and software upgrades, possible naming conflicts in the case of directory merges, and for an inter-forest migration, comparing and unifying the source and target forest schemas.
It is possible you will have to perform the same in the source and extend the schema, too, so you can handle and administer these attributes, eg. skip them, sync them, etc.
DSA will most probably fail if there is an attribute in target but there is no such attribute in source.