If I have the license for NSX 6.2.4 can I use it for NSX 6.4 as well??
Please go through https://docs.vmware.com/en/VMware-NSX-for-vSphere/6.4/nsx_64_upgrade.pdf & NSXv License Versions - Support Insider - VMware Blogs
NSX introduced a new licensing model in May 2016. If you have an active support contract, when you upgrade from NSX 6.2.2 or earlier to NSX 6.2.3 or later, your existing license is converted to a NSX Enterprise license, and you will be entitled to the same functionality in the Enterprise offering. For information about the NSX licencing editions and associated features, see https://kb.vmware.com/kb/2145269
In your case license key will be valid .