Hi @mjh ,
Every time I upgrade my version of OnlyOffice, I replace the default templates with my own. I keep the template files in my own document folder and copy them over the new installation files.
I also keep a backup of the original files, just in case.
I use OnlyOffice on Windows, but this post;
describes a path to one possible location for the template files, which may be a clue as to where to find them on your platform.
Remember to take backups before overwriting any files. And if it works, keep a copy of your modified versions as they may get over-written during upgrades.