1. Back up J1.5 source and J1.7 destination. Use Akeeba Backup.
2. Ensure J1.5 and J1.7 table prefixes match. Use Admin Tools.
3. Log into phpMyAdmin
4. Export J1.5 k2 tables only
5. Import k2 tables .sql into J1.7 database, leave k2 tables in MyISAM format
6. Update k2_items created_by ID to new J1.7 Super User ID by running SQL query:
UPDATE `PREFIX_k2_items` SET created_by = NEW_ID WHERE created_by = OLD_ID
7. Update k2_items modified_by ID to new J1.7 Super User ID:
UPDATE `PREFIX_k2_items` SET modified_by = NEW_ID WHERE modified_by = OLD_ID
8. All of my k2 items had access set to 0 so I made them all public again:
UPDATE `PREFIX_k2_items` SET access = 1 WHERE access = 0
9. Copy k2 media items from J1.5 and paste into J1.7 .../media/k2/
If you don't feel comfortable with phpMyAdmin and SQL queries, you can always have a look at jUpgrade http://redcomponent.com/jupgrade, a component which simplifies the migration of K2 from Joomla! 1.5 to 1.7. More