Sounds like an excellent plan, but it'll be a tall carpet until you submerge it! I forgot to say last time that the submersed foliage is on much shorter 'stems' and usually single lobes, rather than the 4 lobed fronds when grown emersed.
From what it sounds like you're planning, split it apart and plant it emersed, let it grow emersed in your tank and establish and spread and then cut all the emersed growth off, flood and wait for the submersed leaves to form. Take plenty of pictures so we can see how things go!