not hard to migrate your followers, just ask them to follow the new key.
That won't work reliably if you have, say, 800,000 followers.
You'll face a situation a bit similar to someone who has built a following of that size on X and has to, for whatever reason, switch to another platform.
They may be able to persuade a fraction of their followers to come with them. The size of that fraction will depend on the level of engagement they have achieved. If they have a large percentage of highly committed followers, many of these will make the switch, but if they have a high percentage of relatively passive followers with low interest or engagement, they may lose many or most of these.
It could also be compared to losing a large part of your customer / potential client / mailing list in a commercial context.
You'll face a situation a bit similar to someone who has built a following of that size on X and has to, for whatever reason, switch to another platform.
They may be able to persuade a fraction of their followers to come with them. The size of that fraction will depend on the level of engagement they have achieved. If they have a large percentage of highly committed followers, many of these will make the switch, but if they have a high percentage of relatively passive followers with low interest or engagement, they may lose many or most of these.
It could also be compared to losing a large part of your customer / potential client / mailing list in a commercial context.