Hello,
If you still have no quantity in stock and you want to re-set the pre-order available quantity to 100, yes you need to check the "reset" box.
If you refill your product with some quantity (in stock) then the pre-order value will be re-set too.
But if the product quantity stays 0, the pre-order current stock or the pre-order limit won't be changed.
I hope my explanation is clear enough.
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.