I am one that has switched from union to management, albeit not in aviation. So unions are slightly different things in different countries. In my world there is an understanding that the relationship between owners and unions is about checks and balances. A good company needs both, a company doesn't simply exist to make money for shareholders, but also to keep employees securely and longterm employed at a reasonable wage. If a company can't do that, it is bankrupt.
Yes we fight, both ways, we disagree about wages, conditions, strategy, terminations, but both sides (mostly) recognises this is a symbiosis. You are siamese twins sharing vital organs. One side dies, other one dies.
Switching sides works mostly just fine, you take on different responsibilities, but common survival is still the goal.
I know the US is completely different.