freejung - 3:18 pm on Mar 14, 2012 (gmt 0)

I heard somewhere that 301 keeps the old one alive in the SE index

I don't know about Panda-specific issues -- I've just merged a lot of content via 301s but it's too soon to tell what the results are yet -- but I disagree with that in general.

I've moved lots of content using 301s before. Eventually Google catches up and indexes the new URL, and the old URL is removed from the index. That may take a while, maybe longer than it would if you return 404, but eventually the new URL structure is what will be indexed.