Regarding Star-K certified Sabbath Mode ovens:
Rav Heinemann, shlita, stands by his Psak that it is permissible to raise and lower temperatures on YomTov on ovens equipped with that particular Sabbath Mode feature.
Please see Star-K website: http://www.star-k.org/cons-appl.htm, or call our office for details about your particular model. Star-K will, in the next few days, post an audio presentation from Rav Heinemann explaining his views.
For those who wish to refrain from placing their ovens in Sabbath Mode and still use their oven on YomTov, please be aware of the possible serious "Michshol" on many models. Opening the oven door will immediately shut off the heating elements, an act clearly forbidden on YomTov. Thus, even if you don't raise or lower the temperature, it is still important to keep the oven in Sabbath Mode.