Just migrated from Movable Type* to WordPress 4.1.
Which, even with the plugin to help with that, was a gigantic pain in the ass. That plugin wasn’t designed to move 4000+ entries. I ended up having to split my backup file from MT into eight separate files and then import them one at a time.
And I’m not really done yet. This is just the basic template, I need to find something better. Plus it’s still serving all the media files (pictures and such) out of the old MT directories, so eventually that will all have to be moved as well.
Otherwise I had a lot less trouble than Og is having with his upgrade. But I can certainly sympathize.
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* Because of their idiotic “one minute it’s free, the next minute it costs money, the next minute we have ‘pro’ and ‘community’ editions, the next minute it costs money again” marketing model, which is what had me stuck in version 5.