I have not migrated TFS to Azure DevOps Server, I installed the evaluation version of Azure DevOps Server separately so that they would run alongside each other on the same machine and we could evaluate how it performs compared to our old TFS instance.
The problem is that our existing TFS is no longer working and there is no Administration console or anything to suggest it's even on the same machine. Does Azure DevOps Server overwrite older instances of TFS even if you don't explicitly migrate?
It's an expected behavior. We can’t install multiple version of TFS/Azure DevOps Server on the same machine side by side as Visual Studio.
The installer for Azure DevOps may force you to remove the older TFS Application Tier.
Your SQL Server database server will be shareable as long as it's a version that supported by both TFS/ Azure DevOps version.(Details please refer this official Requirements for Azure DevOps on-premises) And as long as both instances have a different database label configured.
According to your situation, doing a dry run of your upgrade in a pre-production environment is strongly recommended.
Detail step please take a look at this link: Do a dry run of your upgrade in a pre-production environment
