Check this thread out for compatibility:
http://gbatemp.net/threads/flashcarts-working-on-3ds-v5-0-0-11.345246/
But it isn't recommended that you update to the new firmware so don't if you haven't already! Read this thread here for the reasons to not update...
Today serebii.net put up several pre-release screenshots for Pokemon Rumble U, here's the link:
http://www.serebii.net/rumbleu/pics.shtml
Here's to hoping this version is an improvement on the previous 2! :unsure:
DS to DS lite didn't change the hardware so new cards weren't needed. DS cards which operate in DS mode will probably never gain access to 3DS mode so new cards will be needed. You should read up on this hack if your interested.
http://gbatemp.net/threads/has-the-nintendo-3ds-been-hacked.339319/