Ya. We've been talking about this in-game recently. There's some talk of starting a new world.... which depending on the transition I'n not sure how I feel about.
Also, 1.9 sounds like it'll be as big of an update as 1.8. So that's something to consider as well.
Apparently the code for generating chunks is different enough where retaining the old world may not be an option. If it was as easy as "kill whatever is unused and regen" I'd be all on the "terrain smoothing team." But I don't know enough about how the chunks are/could be regenned, especially the mega 256x256 versus mini. I'm not even clear about the mini size.