De acordo com isto: link
Veja a tabela chamada "Long Term Service Branch está disponível apenas para a edição do Windows Enterprise": Observe as linhas "Atualização do Windows 10 v2015 Enterprise" e "Atualização do Windows 10 Enterprise com SA²"
Olhe na parte inferior da tabela, em texto muito pequeno: "²Se a SA desistir, terá direitos perpétuos apenas aos atuais LTSBs, com direitos de downgrade"
Eu acho que isso explica muito.
E aqui está uma citação do site que explica exatamente todas as perguntas que eu tinha! (Wow! Deveria ter feito melhor o Google'Fu antes de eu perguntar aqui)
Windows Enterprise edition is available with or without software assurance. Windows Enterprise without SA allows the customer to deploy a point-in-time LTSB release, or previous ones (downgrade rights in other words) and for that release to still be supported for 5 years. Windows Enterprise edition with SA also gives customers the rights to new LTSB release when they become available (every 2-3 years). They can choose whether to install new releases or not. SA also means the customer gains extended support so their chosen release will be supported for 10 years.
Even if a customer does have LTSB requirements, it’s unlikely that all of their devices will require it so customers with Windows Enterprise with SA (or indeed Windows Education with SA) will be able to mix Current Branch, CBB and LTSB according to their needs.
One important point to note is that Enterprise edition without SA will not enjoy updates on Current Branch either. Customers with Home and Pro editions will always get the latest features for the life of the device. Enterprise edition without SA will not. The release that's installed will eventually become out of date and the customer will need to buy a licence again to update.
Windows 10 Enterprise Edition with SA is available through all Microsoft Volume Licensing Programs (Open, Open Value, Select+, MPSA, EA, etc.). You can visit this blog post to read about how to upgrade to Windows 10.