Okay, here is the direct quote from the Pathfinder Bestiary. (emphasis mine)
Change Shape (Su) A creature with this special quality has the ability to assume the appearance of a specific creature or type of creature (usually a humanoid), *but retains most of its own physical qualities. A creature cannot change shape to a form more than one size category smaller or larger than its original form. This ability functions as a polymorph spell, the type of which is listed in the creature's description, but the creature does not adjust its ability scores (although it gains any other abilities of the creature it mimics). Unless otherwise stated, it can remain in an alternate form indefinitely. Some creatures, such as lycanthropes, can transform into unique forms with special modifiers and abilities. These creatures do adjust their ability scores, as noted in their description.
Now, I've had several problems with this rule. The first is that the rule clearly states that you can only assume the form of something within one size category of you. However, there are several monsters that have this ability that ignore that part of the rule, such as certain Titans and most ancient and up dragons. That's somewhat confusing, but eventually I figured out that that only works with creatures that say they can assume ANY form (or ANY of a specific form, such as dragons and medium sized humanoids).
The problem I'm currently having is how many features are removed when turning into another creature. Specifically, I'm looking at the Elysian Titan and how many of its abilities it will loose and keep for becoming a human.
Shape change says that the ability functions as whatever spell is listed in the creatures ability description (in this case, Alter Self), save that it has an infinite duration, and that the creature's ability stats do not change. So, does this mean that they do not gain the stat benefits the spells grant, but still take the size-alteration bonus/negatives that come of casting a polymorph spell on a creature larger than medium/smaller than small (Listed on the table under polymorph), do I have it backwards, and they gain the the spell benefit but not take the negative, or do both stat change get ignored?
*Also, would this passage imply that they keep all of their beneficial, non-sized based abilities, such as dark vision, and scent, instead of loosing them, as is the case when polymorph spells are usually used?