Nuendo 8 will still run if I update to Nuendo 10 right?

I bought it but don’t want to activate until after I finish this project UNLESS I can still run 8 with a 10 license. It has always been this way during my decades on Cubase before, but Nuendo 8 is my first Nuendo.

Absolutely. Your Nuendo 10 license will allow you to continue working in N8, no problem.