This is what i said Marin after looking at cisco asa software matrix. but this gentleman was not very convince so to clear his doubts i run this test on my ASA-X series 9.6 to 9.9 in HA pair the upgrade was a success.
also reading the documentation 9.4 to going to 9.9 is also a zero downtime.
@drlbaluyutkinldy please mark this as answered so other tech guy can find this useful post.
no worries. I hope you find the community supportive and beneficial. we all here to help each other and learn. This is a great place to contribute in order to enhance knowledge. hope you find it helpful. all the best.
Feel free to post any question.