Hi @mahaq. Welcome to the NethServer community. I hope you will feel at home. We will try to help you with your questions.
We have seen a previous request to migrate from exchange to webtop. You might find some hints and tips in that discussion: Migrate exchange to webtop5
For the migration of Windows Server to NethServer. Yes it is possible to promote NethServer as Domain Controller and transfer FSMO roles afterwards, but AFAIK not from the webinterface. @davidep probably can tell you more about this since he was able to add NethServer as a DC to an existing (AD/Samba)domain.
Pro on creating a new domain: you can start fresh.
con: you have to recreate every user, device and GPO.
Please do share your progress so we all can learn!