Upgrading your organization from current versions to Exchange Server SE

Hi, could you please clarify licensing requirements as following:

Can you clarify the license requirements for Exchange Server SE?
(...)

For Exchange Server SE, in addition to purchasing the required Server licenses and CALs, customers must also maintain an active subscription. This means purchasing either:

• Cloud subscription licenses for all users and devices that access Exchange Server SE (for example, Microsoft 365 E3 or E5 licenses); or
• Exchange Server SE Server licenses and CALs with https://www.microsoft.com/licensing/licensing-programs/software-assurance-default (SA).

Other cloud subscription licenses besides Microsoft 365 E3/E5 also satisfy, but we’re calling E3/E5 out because they include Extended Use Rights that provide an unlimited number of Office Server licenses at no additional charge (see the https://www.microsoft.com/licensing/terms/productoffering/Microsoft365/EAEAS for full details).

If you don't buy cloud subscription licenses, then Server licenses and CALs you buy must have Software Assurance.

To summarize your licensing options (choose one):

• Qualifying cloud subscription licenses for users (for example, Microsoft 365 E3/E5) If you go this route, all users that access Exchange Server SE must have an E3 or E5 license.
• License (Server and CALs) + SA for Exchange Server 2016/2019 Maintain SA for usage rights and access Exchange Server SE and updates.
• License (Server and CALs) + SA for Exchange Server SE (once released) Maintain SA for Exchange Server SE for usage rights and access to updates.

I am not sure if I correctly understand this as it is kinda mixed. 

Currently, environment consists of a multiple on-prem servers (CAS+MBX) with Exchange Server 2019 + N * [EXCH User CAL + M365 Business Basic + M365 E3 addon + EOP] -- mailboxes will be still kept on-prem after moving to EXCH SE.

Now, in terms of upgrade path to EXCH SE, do I have to:

- Option A ($$$): Buy Exchange Server SE (L+SA) + N * Exchange Server SE User CAL?  or

- Option B ($): Keep active current M365 E3 addon subscriptions as they make me eligible to install any number of on-prem Exchange Servers without actually buying Exchange Server License?

- Option C ($$): Buy Exchange Server SE (L) + N * Exchange Server SE User CAL + keep active current M365 E3 addon subscriptions?

Option B looks to me that as long as M365 E3 is kept active then I get on-prem EXCH server licenses for free?! Can't be right...

Question no.2 is related to Windows Server 2025 support - EXCH SE is going to be compatible with Win2025 from day one or later introduced by CUn ?

Best Regards,

Tomasz