Mormons:
plural marriage/no plural marriage
no black priests/black priests
baptise Holocaust victims/don’t baptise Holocaust victims
no ecumenism/ecumenism
don’t accept children of gay couples/accept children
book of Abraham provenance and meaning
missionaries not allowed smart phones/now allowed smart phones
sunday meeting times reduced
age and gender adjustments on missionary assignments
Probably a lot more, these are just a few I am aware of.
Edit: sorry those are just changes, not flip flops. I should have read more carefully!