How can one organise moving a large Q&A community?

Question

Imagine a large healthy online Q&A community hosted on a platform developed and maintained by a commercial company. Suppose that the community is increasingly dissatisfied with the direction the company is going, and feels that its feedback is no longer valued like it was 5–10 years ago. Consider that some members of the community want to organise a move along with all user-contributed content. Whatever the cause, it can happen.

Assuming that legal and technical issues are solved (all user-contributed content is available under a permissive license, database dumps are available, features are similar enough): how could users organise moving to another platform?

For the sake of the question, let's take a site with 1k–100k visits/day, 5k–50k questions, and 10k–50k user accounts, of which about 1% (100-500) can be considered highly active members.

One historical example that comes to mind is Wikivoyage and Wikitravel, which still coexist seven years after the split.

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Answer 1

To make a plan, you need to know to things:

1. How many of the leaders / influencers of the group want to depart?

   If you have a significant number, your best bet is to make a stand collectively on the platform, hack the discussions, or even launch a discussion (if you can) on what you feel is wrong. And as more influencers gather around the topic, others will follow, then you'd pitch the idea of a split, and introduce the new platform...

But this scenario depends on the second question:

2. How much control the company has on the platform and what kind of control they exert?

   If the company can and will suppress negative comments, you won't have much options except contacting influencers and ask them to contact others, trying to stay off public publications.
   If they let negative comments be posted you will be able to engage quite a lot of users before anything happens. However, try to keep a personal link to all those who are interested and a list of those, if personal messaging is allowed. When you will propose the new platform, the person from the company moderating the website might not leave it available.