1. Make the miniBB forum a read-only. Move it to a sub-directory, say "archive" (it's presently in the root). Redirect visitors to miniBB urls to archive using 301. Install Xenforo in the root and start over fresh.
2. Import miniBB content to Xenforo using third party converters like gconverter.com. The problem with this is that they don't provide url redirection service. I'll have to take care of that myself. When I asked them whether topic-ids of my old forum and Xenforo will match, they said no, but they'll provide with a table with oldTopicID <=> newTopicID relationship.
My miniBB urls are in the format:
Can anyone help me with creating redirects?