The wallet 2.1.3 can not be installed on top of the old wallet. If this is done, the wallet will not finish synchronizing and will not show a balance, and the synchronization will be 1 block ahead. Only by completely deleting all the data of the old wallet you will succeed, you only need to insert the key.
It depends, It can be installed on the top of the 2.1.2 version.
From the old version (the ugly one) instructions are here :
http://ird.cash/update-to-new-gui-wallet. In any case, backup your wallet(s)