Migrate FRS to DFSR Windows 2012 R2 DCs, 2008 DFL, 2003 FFL

Hello,

We have an AD environment with 4 2012 R2 DCs running at 2003 Forest Functional Level and 2008 Domain Functional Level. It's a single domain forest.

We have some legacy NT4 and Windows 2000 clients. Because of this we haven't patched the DCs with security or Windows updates for over 2 years (we've been told by Microsoft that patching the DCs will break legacy interoperability).

I'd like to migrate from FRS to DFSR by following the links below

https://techcommunity.microsoft.com/t5/storage-at-microsoft/sysvol-migration-series-part-1-8211-introduction-to-the-sysvol/ba-p/423456

https://techcommunity.microsoft.com/t5/storage-at-microsoft/sysvol-migration-series-part-2-8211-dfsrmig-exe-the-sysvol/ba-p/423470 https://techcommunity.microsoft.com/t5/storage-at-microsoft/sysvol-migration-series-part-2-8211-dfsrmig-exe-the-sysvol/ba-p/423470 However I have a few questions:

1. Is there a risk of breaking legacy clients\apps when migrating to DFSR?
2. Is it OK to use 2003 Forest Functional Level with 2008 Domain Functional level?
3. Is it OK to run the above steps on 2012 R2 DCs?

Even though we have a small number of DCs, we have over 7000 users in a critical healthcare environment, hence I need to proceed with caution.

Thanks in advance