How to notify purchasers of updates on different platforms

I have a 10k story that has been out for around a year. Many readers expressed an interest in a sequel, but I have decided to update it so that it remains one file, just 2.5 to 3x longer. I know KDP will, if requested, send a notification to purchasers so that they can download the new file. Similarly, Smashwords onsite sales give the purchaser the option to DL any version of the file that has been uploaded since their purchase. (Is anyone aware of Smash sending out notices?) The title also is available on B&N and Kobo.

I will of course make an inquiry with BN/Kobo - but does anyone know whether they do this, if the request needs to be sent to other than regular support/help and if the request identifying the title is enough or significant justification has to be made?

I know I could release a standalone sequel free, etc., but don't wish to do so because, among other reasons, I've also added more to the portion already published. Plus Amazon isn't guaranteed to price match, particularly when a title sells well enough on the store.

Thank you for any insight you can offer.

Christa