Libgda's reports API has not changed, but the syntax of the XML nodes specific to Libgda (those nodes are replaced by contents when the report is generated) has changed:
The parameter name to access a data model's column name by its name is now <query_name>|@<column_name> instead of <query_name>/@<column_name>
The parameter name to access a data model's column name by its index is now <query_name>|#<column_index> instead of <query_name>/#<column_index>
The parameter name to access the total number of rows of a data model is now <query_name>|?nrows instead of <query_name>/%nrows