Mass Effect Forums have an explanation from Mass Effect 2 Lead Designer Preston Watamaniuk on why you will not be able to replay the game with the same built-up character after completing BioWare's upcoming RPG sequel, leading to a corresponding change to how insanity mode will be handled, as explained in a forum post by Lead System Designer Christina Norman (thanks 1up). Here's word on the change:

The reason is progression. We have been working very hard to make sure we design the abilities system to offer smooth progression into ME3 from ME2. Allowing double progression on characters makes that almost impossible. We have to have reasonable knowledge about where a character could end up finishing all content on a playthrough. We also want to offer choice of character build within specific classes.

We replaced that feature with playing after you were done because it preserves progression and allows for smoother downloading of PRC.