Hi,
You can do something like:
UPDATE #__hikashop_product_category SET category_id = 10 WHERE category_id = 20;
Just replace "10" by the new category id and "20" by the old category id.
It would "move" every product in the category 20 to the category 10.
Regards,
Jerome - Obsidev.com
HikaMarket & HikaSerial developer / HikaShop core dev team.
Also helping the HikaShop support team when having some time or couldn't sleep.
By the way, do not send me private message, use the "contact us" form instead.