Download links dont work on woocommerce

I use woo-commerce 1.6.6 ,
I sell downloadable products , my download links used to work, but somewhere between the update of woocommerce and wordpress they now only show 404 error. links that used to work….

I read somewhere a suggestion to change my permalinks setting to :
Wordpress Permalink Settings – custom structure > /%category%/
but I’m afraid to do that since i have an old blogger blog directed into my new site, based on a month and name structure :
http://www.yooladesign.com/2013/01/sample-post/

I’m worried that if I change the link structure I will loose that, so i cant risk it.

Does anyone have another suggestion to solve this ?
Its a real pain to have all your customers sending emails saying the download link show 404 ….

Thanks !
p.s.
this is ticked on woocommerce pages tab : Product base page Prepend product permalinks with shop base page (shop)