It's actually not difficult to answer your question. Changing to the current B4A version 12.8 (64Bit) does not actually represent a risk as far as updating the development environment is concerned. Many new functions have been added to make programming easier for you.
I see a much bigger problem in the further development of the Android operating system itself. Depending on the SDK level with which you have developed your applications, there are many new restrictions on the part of Google, so that you may have to change your programme code considerably in order to meet these new requirements of the operating system on the end device. This applies at least to Android on mobile phones or tablets. I think the same applies to smartwatches if they run on this operating system.
Nobody will give you a guarantee that an old app will also run on modern devices, recently there are even minimum requirements for SDK levels, which means you have to programme your application for a minimum SDK level in order to be able to offer it in the Playstore, for example.
However, your old programme code should also work in a new development environment. Pay attention to notes in the IDE if you load an older programme into the IDE.