For double beta decay, it wouldn't change anything, since neutrinos are not Majorana particles in the standard model. For cosmology, it would suggest something

For double beta decay, it wouldn't change anything, since neutrinos are not Majorana particles in the standard model. For cosmology, it would suggest something about our understanding of the big bang or early universe is wrong. I'm less familiar with cosmology, however, so I'm not sure in what way things might change. We'd need a new way to explain why matter is less clumpy than it should be due to the massive neutrinos, or why there weren't as many neutrinos then as we think there should have been.