I'll reiterate, enhanced limited lifetime warranty provides lifetime software update entitlement. It is only applicable to the designated switch models covered. It is true that it only provides 90 days of JTAC support, but you are not performing an "illegal" activity by upgrading the software beyond 90 days.
https://www.juniper.net/support/warranty/990240.pd
Limited Lifetime Software Update Entitlement:
In addition, throughout the period that it warrants the Covered Hardware Juniper Networks shall make available to Customer for use solely on the Covered Hardware such updates of the Software as it may release for general availability, provided that availability of software and updates shall be subject to the standard Juniper End of Life/End of Support guidelines, as posted and in effect on the date that from time to time; provided, further, that updates shall be subject to the terms of the version of the Juniper Networks form End User License Agreement (“EULA”) applicable to Junos operating system embedded software and in the form posted on Juniper’s public website as of the date that the update is retrieved or delivered to the Customer. (Note: as of January 2011, the URL for the EULA revision that is current as of January 2011 is http://www.juniper.net/support/eula.html.) This is not applicable for products sold through Juniper Certified Pre-owned Program that has passed Last Order Date.
https://www.juniper.net/support/warranty/990240.pd
Limited Lifetime Software Update Entitlement:
In addition, throughout the period that it warrants the Covered Hardware Juniper Networks shall make available to Customer for use solely on the Covered Hardware such updates of the Software as it may release for general availability, provided that availability of software and updates shall be subject to the standard Juniper End of Life/End of Support guidelines, as posted and in effect on the date that from time to time; provided, further, that updates shall be subject to the terms of the version of the Juniper Networks form End User License Agreement (“EULA”) applicable to Junos operating system embedded software and in the form posted on Juniper’s public website as of the date that the update is retrieved or delivered to the Customer. (Note: as of January 2011, the URL for the EULA revision that is current as of January 2011 is http://www.juniper.net/support/eula.html.) This is not applicable for products sold through Juniper Certified Pre-owned Program that has passed Last Order Date.