For the best Lync / Skype for Business login experience it is important to keep the sign in address, e-mail address and user principal name the same. For Lync / Skype for Business and Exchange purposes the four attributes that are important are mail, proxyAddresses, msRTCSIP-PrimaryUserAddress and userPrincipalName.
When a person’s name changes a lot of organizations implement different strategies of keeping relative addresses in sync.
