While it may be possible to convert simple feature classes from 9.0 to 8.3, this is an unsupported procedure.
ArcGIS 9.0 geodatabases contain many new datatypes that did not exist in the 8.3 schema. Adding these new datatypes could possibly corrupt your 8.3 geodatabase.
From the ArcGIS Desktop Help:
"You cannot check-out from a 9 geodatabase to an 8.3 geodatabase since 9 geodatabases support data types that 8.3 geodatabases do not."