Both .fbz and .fbr files are used in a tool called BB FlashBack, so if any kind of fbz to fbr conversion is even doable, it would be certainly done with this software. It is certain that latest versions of BB FlashBack tools no longer support FBZ format and it is not possible to open these files with it.
Perhaps it is related to making new recordings based on screen recording in some of the earlier incarnations of the software, but we did not find any mentions about this conversion. And FBZ format is not even mentioned in latest BB FlashBack help files.
In case you own .fbz recordings and need to export them to modern .fbr files, it might be best to reach out to Blueberry software support for assistance.
Updated: October 26, 2023