There is no such thing as "replacing the template" of an existing file. A new Inventor file is essentially just a copy of the template that existed at the moment when it was created, and has no link or connection to the template from which it was created.
One tool that might be useful (or might not) is Design Assistant. DA includes a tool called Copy Design Properties.
You can start DA by right clicking on the source file (the drawing template, in your case) in Windows File Explorer and selecting Design Assistant. In the DA window, under the Tools menu, select Copy Design Properties. This allows you to select which iProperties you wish to copy, and then select the files to which you want to copy them.

However... this applet is very old. The GUI has not been updated in the twenty years that I've been using Inventor, and lacks even the most basic controls such as resizing(!). You can't filter by file type, and the choices of files to present for selection is limited to Related Files, Current Folder, or SubFolders. Pretty clunky. But, it works.
Best strategy for using this tool would be to have all the target files in one folder.
Hopefully, someone else has a better idea.
Sam B
Inventor Pro 2023.1 | Windows 10 Home 21H2
