That warning message will only display once for any given program or app. You will see it again for BIAB if there is an OS update.

As far as the "free upgrade", I think PG Music's general policy would be "NO". I think they still offer a 30-day full refund policy, so if the 2019 version was released within 30 days of your purchase, you could probably "refund" the 2018 version and purchase the 2019 version. However, they don't announce in advance when the 2019 version will be released, so you're at risk that this could be beyond your 30 day window. I would really encourage you to contact someone in sales with PG Music about this. Or else, I might suggest to play with this version for a few weeks, then return it for a refund, and wait for the 2019 version to release.

good luck

Macbook Air
1.8 GHz Intel Core i7, 4 GB RAM
OS High Sierra
BIAB for Mac 2018