Starting from I don't remember which version and up to v.1.12 there was a nice (for me) conversion feature in Calibre: i.e. checking "Filter Style Information/Fonts" while embedding a custom font family would have resulted in removing all old fonts AND embedding the chosen font family.
Now, in v.1.13 this feature is gone.
I don't know whether it is a bug or a feature, but I'm again rolling back to the previous version.
Now, in v.1.13 this feature is gone.
I don't know whether it is a bug or a feature, but I'm again rolling back to the previous version.