I'm a long time user of Limesurvey (from the phpsurveyor days!) and really like this software.
Unless I'm wrong, it seems to be impossible to reimport old answers from a modified survey. I needed that option, so I wrote a python script to do this work. It connects directly to the MySQL database and doesn't do any kind of sanity checks. Use at your own risks!
What I do is modify the old survey DB on phpmyadmin, adding blank/dummy columns or deleting them on the "Structure" tab, according to the changes made comparing side by side each DB, then reload the old DB on the limesurvey admin page on the corresponding survey... The only problem I've found doing this is that ID numbers change due to export, but no biggie for me (Still need to find how to avoid this, but later...). Usually this takes me a few minutes to do and I do this often (New ideas pop up unexpectly some times).