I am not on discord, so I don't have skin in the game. But it depends on the community I guess. If everyone stayed on discord then there would be no change. But any kind of change would probably have some kind of effect, even if all people migrated to a new platform.

I can try to think of a simple scenario. For whatever reason, a user may not be as active in the new platform as they were on discord. This could alter the community dynamics. On a bigger scale this could have visible effects in the actual community.

To be clear, I am not advocating in favor of staying on discord. I just find the concept of community building interesting.