I can't find the information on when Google Play will not allow updates that use 1024 bit certificate. There is a good chance they may continue to allow updates, but won't allow new submissions, but you may need to try submitting an update in order to find out.
Building an APK with a 1024 bit certificate has at least two problems. One is that Java already dropped support for it, and also the current Animate doesn't support it either. You can download Animate CC 2017 from the CC menu to get around the Animate part, and read this article about getting around the Java part:
Packaging Android app fails -- Java 8
If you are able to do an update with the 1024 bit certificate, but you don't want to have to keep Java 6 and Animate CC 2017 around forever, you could think about building in promotion for a new version of your app, that would be 2048 bit certificate from the start. It would be a new, you would start over with download count and with reviews, but hopefully most of your current users will install the new app and uninstall the old one.
Animate CC 2018 has the 1024/2048 menu grayed out, but using AIR 27 in CS6 still lets me choose 1024. Animate CC 2018 lets me choose a certificate from 5 years ago, which is likely to be 1024 bit, maybe CC 2017 isn't needed unless you need to generate a 1024 bit certificate. I'm not sure.
For what it's worth I think doesn't affect the Amazon Android App Store, Amazon ditch your certificate anyway, and use their own one.
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