I've used Quicken for many years. Since my spouse passed away, many of my current categories have changed or are no longer needed. I still want to have access to my existing data file, but would like to clean things up a bit. Thanks!
Instead of starting a new data file, why don't you consider just hiding those now unneeded categories?
Once a new file is started, it's just this side of impossible to put the 2 back together and there's trivial, at best, performance gain from the action. Additionally, if you want to compare current file to info in prior years that would require exporting to a spreadsheet and Q can't compare data across multiple files.
The biggest downside is that you now have TWO files and when researching something a few years from now you have to remember "Was it before the split or after" so that you open the correct file.
I would just hide the accounts and categories that are no longer needed and continue with the single file.