Hay Gogeta,
Yea, I misunderstood you. Yes you are correct, if you delete the retention category it will get an "uncategorized" retention category. What I would do if I was you is simply rename the retention category and give it the new retention period.
If this is still not a workable solution, I am playing with Archive Shuttle http://www.vcare-it.com/, which will re-stamp a new retention category on the items. I should have a review of it soon on evdiscuss.net, I am just madly busy on a massive design.
All in all you have loads of ways to resolve this issue. If I was you I would not delete the retention category, I would simply rename it to something more suitable.