I have the solution!
Ok, so I had the same problem than you.
My girlfriend and I were sharing my apple account for the appstore until iOS 8.
She had an iCloud account that she used for her personal services like Calendars, Reminders, etc.
With the release of iOS 8, we changed her iPad in a way that there was only her account connected on it and using the family sharing.
But we rapidly saw that she couldn't update her apps. I successfully downloaded a free app but there was no way that she updated apps that had been downloaded when we were on the same account and she also could't access to my purchases.
The solution I found was going to the family sharing preference pane and touch the section with our names.
Here, I selected my girlfriend's account who was just an user (when I'm an organizer). And I put it as a parent/tutor.
I restarted the device.
After that, I did a verification... The problem wasn't solved yet. But I then went on my girlfriend's iPad on the same section and discovered that she had to validate her account. I set up her Apple ID and restarted our two iPads. So, with the device of the user that has the problem, just go in the family sharing preference pane, touch his account and fill-up the blanks with his Apple ID/Password.
When they were on, I went on the App Store on her iPad and checked the purchase section. MIRACLE, she could access to my purchases!
I, then tried to update an app but it failed. However, the error message was different, it told me that we needed to have a payment option.
So I went on App Store/Account/Payment Informations (maybe the translation isn't exact) and just filled my credit card informations once again and saved
Restarting our iPads.
Going to the Update section of the App Store and trying to update one... SUCCESS!!!