Yes. The same license can be used for both 5.0 and 5.1
In order to perform this upgrade there are a couple patches that need to be installed before upgrading to 5.1
1) ACS 5.0 patch 9. On CCO: 5-0-0-21-9.tar.gpg
2) ADE-OS version 1.2 /// upgrades operating system version. On CCO: ACS_5.0.0.21_ADE_OS_1.2_upgrade.tar.gpg
Both these steps use the following command "acs patch install patch-name.tar.gpg repository repository-name "
Then can perform the upgrade to 5.1 using following command:
application upgrade application-bundle remote-repository-name
All the patches/upgrade bundles can be downloaded from CCO. 5.1 package is called "ACS_5.1.0.44.tar.gz"
More detaield documentation is at:
http://www.cisco.com/en/US/docs/net_mgmt/cisco_secure_access_control_system/5.1/installation/guide/csacs_upg.html#wp1167547
