Apart from recompiling the code for 64 bit (32 bit apps are no longer supported since macOS Catalina), there has been extensive work to update functionality that relied on OS X APIs that Apple has discontinued or modified with incompatible behaviour.
Other changes are related to adjusting to the evolving macOS security constraints and to including some of the recent emulator changes that have been released in the Windows version. A few of these changes have also been applied to the 32 bit app (version 1.3.1) to make them available on older versions of OS X.
There is a new Quick Start Guide to help users new to Q-emuLator, or even the QL:
If you purchased a registration for version 1.x, this is a paid upgrade, except for people that have registered in the last year. (If you've registered in September 2019 or later, you may find that the new version already accepts your existing code, or contact me if it doesn't).