Not everybody can in this life but some for sure and totally.

people can't become a completely different personality (or if they can, the personality they become is still related to their old one or their core in some way), but they can change in the meaningful ways that they should change in.. if they really want to, or if they're graced, or maybe just some people can..

It's really in your flesh and blood, you can't change until you decompose. there are practices that includes mass level of body recycling while alive, or burning your entire karma( just a term) , until you basically don't remember much of your past. Doing so can be dangerous aswel cz you gonna lose your centre motivation, and unless you can build something new you'll go mad, I don't know how much time it takes to build it, could be 7 years or more ( source inner alchemy).

People can do a complete 180, it's a matter of willingness and being open - it's difficult, especially as you age and become set in your ways, but it's possible