No you don't need to.
When switching from Dalvik to Art. The new runtime just pre-compiles the applications boot up sequence. Sort of like odexed systems but more intuitive and less lag is present.
This does increase the size of apps and uses up a tad bit of extra space then you had on Dalvik.
No need to be worried, your experience with ART will be better.
Again let me answer your question - You do not need to redownload your apps.