It may be useful to keep track of what terms mean or at least state a definition. Wikipedia defines probability as follows:
Probability is a measure quantifying the likelihood that events will occur.
In the case of being pregnant or not, the current measure of the event of being pregnant is 0.5. That is all that number refers to. It is the current measure of the event of being pregnant.
Eric Schwitzgebel describes belief as a propositional attitude. In particular:
Contemporary Anglophone philosophers of mind generally use the term “belief” to refer to the attitude we have, roughly, whenever we take something to be the case or regard it as true. To believe something, in this sense, needn’t involve actively reflecting on it: Of the vast number of things ordinary adults believe, only a few can be at the fore of the mind at any single time. Nor does the term “belief”, in standard philosophical usage, imply any uncertainty or any extended reflection about the matter in question (as it sometimes does in ordinary English usage). Many of the things we believe, in the relevant sense, are quite mundane: that we have heads, that it’s the 21st century, that a coffee mug is on the desk.
He describes knowledge in terms of belief as follows:
Much of epistemology revolves around questions about when and how our beliefs are justified or qualify as knowledge.
Finally when something changes, that change need not be linear or a total ordering of events. Although an increase or decrease is a change, change itself may involve a more complicated ordering of events such as moving from one side of the room to the other or changing one's mind about the likelihood of an event.
Consider the question in the title:
Does our knowledge increase or decrease when we assign 0.5 probability to our belief, after being presented with new evidence?
That knowledge changes does not mean that such change is part of a linear ordering. Knowledge need not be increasing nor decreasing. Probability is a measure assigned to an event. Belief represents a propositional attitude about the probability of that event. Part of the justification for the new belief comes from the new evidence. Her current belief is that the probability is 0.5 that she is pregnant, not that she is or is not pregnant. She doesn't know now nor did she know previously whether she was pregnant.
Schwitzgebel, Eric, "Belief", The Stanford Encyclopedia of Philosophy (Summer 2019 Edition), Edward N. Zalta (ed.), forthcoming URL = https://plato.stanford.edu/archives/sum2019/entries/belief/.
Wikipedia contributors. (2019, May 30). Probability. In Wikipedia, The Free Encyclopedia. Retrieved 14:03, June 9, 2019, from https://en.wikipedia.org/w/index.php?title=Probability&oldid=899504762