update text format "filtered html" on old nodes
Update the database directly using SQL queries. The queries are something like

UPDATE field_data_field_[example] SET field_[example]_format = '[text_format]';
UPDATE field_revision_field_[example] SET field_[example]_format = '[text_format]'


for a field named field_[example] that was created with the Field UI module and a text format with machine name [text_format]. Clear the cache afterwards.