Welcome to the Forum. It sounds like you are a quite experienced user of S&T.
Quote:
Originally Posted by placasse47
...Is there a way to apply the new POI Megafile template to existing routes by default ?...
I don't think there is a default way of doing what you want to do.
What you might consider doing is to export your existing routes one by one as GPX files and then import them, again one at a time, into the STT file.
I might also point out that we can expect further POI Mega files as people add more and more features. Therefore, an STT file you create today might be superceded by newer STT files that incorporate more POIs. This might create further complications, depending on how you are using your files.