Can Use 9ah Batteries Instead Of 7ah Batteries In The UPS

some months ago my UPS batteries died on me after a little more than 1 and half years and then i replaced the batteries from the same store few months ago. The batteries that came in the UPS were 12V 7.5ah (each) and those guys convinced me that 12V 7ah (each) batteries will work fine too.. now a couple of days ago i had a power-failure and the UPS kept the PC hardly 8mins and suddenly started beeping fast and shut off everything even the LCD was running on lowest contrast and brightness and only windows was running so there was no peak load as well on PSU and the last batteries used to give around 15 to 20 mins easily.So now i know those liars sold me out the low-quality chinese made batteries which cannot even give much backup time and who knows if they're refurbished? they did'nt came with any warranties. Now my real question is im looking to buy some better qualities Lead Acid Batteries for the UPS and i was think if i might be able to use 12V 9ah batteries instead of 7ah or 7.5ah.. i heard that the 9ah will hold more charge and can give some more time for backup (just in case) will they work?