I don't know if this is possible but why not let players keep the items they get and reset the levels. I find it to be a good solution. ( I mainly would like to keep my legendary gear from the iron banner that would effect the gameplay because it is cosmetic)
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I think that would be best for everyone. Let beta testers full their vault with all the items they amassed during the beta and reset all characters. The vault would be waiting for our new characters on 9/9.