Existing apps
1. You will not gain anything from switching to AAB.
2. You can switch by following Google instructions with the pepk tool. It is really not too complicated.
New apps
1. For now you can still ignore AAB.
2. If you do want to use the new AAB then you will need to create a new RSA signing key:
c:\java\jdk-11.0.1\bin\keytool -genkey -v -keystore new-key.keystore -keyalg RSA -keysize 2048 -validity 10000 -alias b4a
3. You will also need to add:
#SignKeyAlgorithm: -digestalg SHA1 -sigalg SHA256withRSA
It will be simpler in the next version of B4A:
1. New keys will be created with the RSA algorithm.
2. The bundle step will auto detect the algorithm.