Option 2 will work. When you install IPAM 2.0 it will prompt you to enter your new license key and it should automatically let you add up to 16000 IP's afterwards.
Hi HHernandez. If you are not prompted to enter your license during the upgrade. You can use the License Manager. It may already be there on your server at c:\program files\solarwinds\License Manager. If not, it is available from your customer portal on the license page. You can find the download link there too also. You can then click the 'Upgrade' link beside IPAM and then enter your details.
Below is a guide to registering your license. it is written for NPM but is the same procedure for IPAM also.