I normally use SW2018.

I downloaded SW2019, opened a few assemblies, and then left it alone. Lesson learned... I'll wait until SP1 to actually use it.

An assembly I saved in SW2019 won't open now in SW2018, and I need it to. Is there any way to go back in SW2019 and save it with compatibility with SW2018?

I was under the impression this problem was fixed a few versions ago!