Last year, my mom was upgraded from Elite class to Royal/Laurel YYZ-TPE. On the return journey, she was joined by my sister. Both were to sit in Elite.

In TPE, EVA offered my sister an upgrade from Elite to business, but my sister said she wouldn't leave mom behind, and that they could give it to someone else.

The check-in agent saw additional space and upgraded both. Interestingly enough, my sister and mom had no prior status.

I think fare class is important and it also helps if Elite class is oversold (which ultimately, was the case), so the time you check in might also be a major factor in landing a comp.