In the Forum Manager set the Default View Age to 'Show All Threads':
Admin CP -> Forums & Moderators -> Forum Manager -> Edit Forum -> Default View Age -> Show All Threads
Or to update them all at once, run this query:
UPDATE forum SET daysprune=-1;
After running that query, update the forum counters:
Admin CP -> Import & Maintenance -> Update Counters -> Rebuild Forum Information
Admin CP -> Forums & Moderators -> Forum Manager -> Edit Forum -> Default View Age -> Show All Threads
Or to update them all at once, run this query:
UPDATE forum SET daysprune=-1;
After running that query, update the forum counters:
Admin CP -> Import & Maintenance -> Update Counters -> Rebuild Forum Information