Yes, the purchase of RM7 will include a free RM8 key.
Make sure to do a complete uninstall before upgrading to Big Sur and installing RM7 again. When RM8 is in community preview or full release it will be a separate program. You won't need to worry about the uninstall of RM7 unless you want to.
I don't regret purchasing a RM8 key today to see the full power of the preview... but I do want to express my disappointment in that I have the very last 2013 mac that CAN'T UPGRADE to Big Sur since it is not one that has the retina in it. Catalina is by no means an old operating system... but I am going to be totally left behind on my macbook pro that does 99% of my genealogy.
I believe in RM you have Great software and I've been a very long time customer. But knowing I am going to be left behind when we were promised something for so long... I'm very sad.