If you don't change the default search engine, then updates to the default search engine will affect you. If you explicitly change the default, then the updates should leave your choice intact.
As commented elsewhere: not true. I had to change settings (back) on quite a few accounts, so I'm certain of it. And yes, I did have StartPage as default.
But you've said what s/he said - you liked the default, but the default changed. That's what you both agree on. Had you not remained on the default search engine, your move would have been retained. This leaves the question of whether a past change first away from the default and then back would have been counted as a choice to be retained or as merely the default.
What kind of convoluted reasoning is behind this antifeature ? Please do not alter or change user space. Please consider how this kind of thing impacts the life of the many people who struggle with computers.
Understand that this entails on what's left of trust one could still have with mozilla.
Or don't as I'm personally not affected and already have grown a deep mistrust for mozilla.