"But what will I tell my neighbors?"
Once someone makes a decision about your cause or your product or your resume, it's almost impossible for you to persuade them that they were wrong. You're no longer asking them to remake the first decision, you're asking them to admit an error, which is a whole other thing.
Compounding this, organizations often make it awkward for someone who is trying to come around to be embraced, largely because the tribe is hurt that they were rejected in the first place.
The opportunity is to encourage the non-supporter to look at new information and make a new decision. Give them the story they need to tell their colleagues. "Well, I know that I always thought this brand was a cult and I said I would never use them, but then I saw their new product line. They've listened to all the stuff I said was wrong and fixed it…"
And step two is to celebrate the newcomers, not to dredge up their past statements and wave them in their face.