Ok, first of all, it was reduced to a 23 day sentence. That wasn't speculation. Look up "paris hilton 23 day" on google, and a number of news sources will confirm that. Second, I agree that having the sentence reduced for "good behavior" was inappropriate, but once it was done it should not be reversed.
Third, I think it's obvious why the sheriff re-assigned her.
Actually, all the reliable news sources(CNN, Fox News, AP) I've seen indicate that her sentence was not reduced to 23 days.
Hilton, 26, was originally sentenced to 45 days in jail for violating probation in an alcohol-related reckless driving case, but had been expected to serve 23 days because of state rules allowing shorter sentences for good behavior.
I also found another site that indicated part of the good behavior 'rules' are actually California state law and that things like her showing up for court dates helps her. Another note of interest is that Sheriff Lee Baca supposedly received a $1,000 donation from a relative of Paris sometime last year, and hes that one that released her so everything isn't as "clean" as one might hope.
I still haven't seen anything that indicates that Paris will have to serve the full 45 days in jail now, and afterall if the good behavior rules are infact a law then the judge will not be able to override them and she'll get out after 23.