Hello @Andreas ,
Thank you for posting here.
Here are the answer for your references.
Q: If I raise my forest functional level and domain functional level to 2016 would that cause any issues with servers and clients in the domain that is Windows 7, Windows Server 2003 and 2008 ?
A: From the link below (Forest and Domain Functional Levels), we can see:
Functional levels determine the available Active Directory Domain Services (AD DS) domain or forest capabilities. They also determine which Windows Server operating systems you can run on domain controllers in the domain or forest. However, functional levels do not affect which operating systems you can run on workstations and member servers that are joined to the domain or forest.
Meanwhile, the highest domain/forest functional level is Windows server 2016.There should be no issue with the servers and clients in the domain that are Windows 7, Windows Server 2003 and 2008.
As a kind of reminder, perhaps the applications on workstations or member servers (such as Windows 7, Windows Server 2003 and 2008) may be impacted by forest functional level and/or the operating system version of domain controllers.
For example:
Whether specific Exchange version can be supported, it depends on server operating system version installed with Exchange, Exchange version and Active Directory environments (including DC operating system version and AD forest functional level).
Exchange Server supportability matrix
https://learn.microsoft.com/en-us/Exchange/plan-and-deploy/supportability-matrix?view=exchserver-2019
The link below can help you understand the issue of domain /forest functional level and operating system version of domain controller better.
Forest and Domain Functional Levels
https://learn.microsoft.com/en-us/windows-server/identity/ad-ds/active-directory-functional-levels#windows-server-2019
Hope the information above is helpful.
Best Regards,
Daisy Zhou