Ukraine was already in a civil war past 2014, and most people in eastern Ukraine are ethnically Russian or have family ties to Russia. They do not see this as an invasion. A few slides from this lecture that Mearsheimer gave back in 2015 clearly show the dynamics in Ukraine. First, here’s the demographic breakdown of Ukraine:
As we can clearly see from the voting patterns in both elections, the country is divided exactly across the current line of conflict. Furthermore, a survey conducted in 2015 further shows that there is a sharp division between people of eastern and western Ukraine on which economic bloc they would rather belong to:
So, the most likely scenario is that people from western Ukraine would’ve fled to Europe, while many in central and eastern Ukraine would’ve gone to Russia.