Foolishly left old beta on my iPhone that has now expired, is my only option to restore and lose everything? I installed an iOS 8 beta on a test iPhone I have back in the summer, and forgot to keep it updated with more recent betas, and now it's not allowing me to get into it due to the beta being expired.

Is my only option to be able to use it again to restore it from scratch and lose everything? Or is there some way to update it to the actual released version of iOS 8?
Simply download the IPSW of the final build of iOS 8.1 from developer.apple.com, connect your device to iTunes, alt-click the Update button and select your IPSW.

No data will be lost and your device will be updated.