Ex post facto laws under the U.S. Constitution are barred only if they make a conviction possible where the conviction would not have been previously possible, or where the punishment for the crime is increased after conviction.
The problem you face is that the retroactive application of the cited law does not increase your punishment, in so far as your liberty interest is concerned. That is, you will not be forced to serve more time in prison -- you will only be subjected to a notice on your drivers license.
Louisiana has pretty liberal morals for the South, but when it comes to sex offenses, courts are extremely reluctant to do anything that might give a prior offender the opportunity for new bad acts.
So, unless you are prepared to take this battle all the way to the U.S. Supreme Court, I think you will have some difficulty prevailing. However, the judge gave you the restraining order to begin with, so that certainly gives you an edge at the start. Maybe it will be the State that is appealing and you will consequently be protecterd during the appeal process.
As for the ultimate outcome, it could depend on the make up of the High Court, when your appeal gets there. We will have a new President soon and likely Justice Stevens will retire in favor of a new appointment -- whose disposition in criminal matters is unknown but may be highly determinative when the time for a decision arrives.
So, at this point, it's a crap shoot, in my view. Certainly some attorney will want to prosecute this appeal, because it has a lot of media potential.