I plan to merge two Invision communities:

• Community A - 75K members - Source
• Community B - 25K members - Destination

My question is:

Will members of community A be able to log in with their usual user/password combination, or will those be reset? I'd instead prefer to find a way for them not to have to reset their password.

Correct. Passwords will transfer with the user except in events where the user is a member in both communities. In that case, the user's content is merged into the Community B user and that user will need to login with their credentials used in Community B.

And one other question, @Jim M so I will not open a new thread:

Is it possible to redirect the old thread URL to the new one? Because I think the thread number, it will change. This will help me a lot with SEO

The topic ID will indeed change. I'm afraid, there is no method currently to redirect this.

Talking with one of our developers, there is a lesser known, not really documented way to redirect this:

You redirect the entire old domain to <new-url>/ic-merge-X/*
Where X id the id of the 'core' merge.

That way your old topic IDs will be handled.

 

Talking with one of our developers, there is a lesser known, not really documented way to redirect this:

You redirect the entire old domain to <new-url>/ic-merge-X/*
Where X id the id of the 'core' merge.

That way your old topic IDs will be handled.

@Jim M Can you please help me with the Nginx rule that I should set up to handle all merged topics?

 

We would not provide rules for nginx. You would be best to contact your hosting company for some assistance with setting up some rules for you