Crypts are often grown this way--it's one of the main causes of 'melt', where you buy a pretty plant, put it in your tank, and all the leaves melt. If the root structure is intact and healthy, it will regrow leaves and be fine, although the leaves may be different size/color from the emmersed ones.
Presumably, going in the reverse order runs the same risk--the leaves that grew immersed may die off, and be replaced so long as the root structure is healthy.
Thanks for the info! I'm tinkering with an "algae scrubber" prototype for my 34G tank...
I'm putting it together today. You know, make something grow really well away from the display tank so that this plant soaks up extra nutrients... hopefully this mean less algae in the main tank...
There are loads of options if you're looking for something that doesn't need substrate. Pothos and spider plants come in a wide variety of colors and will be more than happy to grow with their roots in the water.
Oh thanks for mentioning other plants. I went to Aquarium Adventure looking for floating plants but they didn't any. He recommended the crypt after I told him what I wanted the plants for. I'll post my crazy contraption later.