Is it possible to convert our existing projects/collections to this template?
I am afraid it is impossible to achieve that.
According to official documentation, we could to know:
You can change the process a team project uses from a system process
or inherited process to an inherited process. You can only change team
projects to use another process that inherits from the same system
process. That is, you can change an Agile-based team project to any
process you created from the Agile system process as well as to the
Agile process. Whereas, you can't change a Scrum-based team project to
an Agile-derived inherited process.
So, it is not possible to convert process templates for your existing projects/collections in Azure DevOps Server.
You could create a new projects/collections based on SCRUM and then move your source code and workitems to that new projects/collections. For source control you could always branch out from a stable version, and keep the old project around. That way you will keep you source history. For work items, you can export them to Excel, create a new Excel connection to Azure DevOps Server, that is connected to the new team project, and then copy the workitems and pushing them from the new Excel file into the new project.
BTW, you can learn to use witadmin
to change things, but that isn't a best practice if you have workitems in the team projects:
Check this thread for some details.
Hope this helps.