I think the discussion about rules and timing for rule changes is good and important but missing a key aspect: Consider that the student may have spent a day's work or possibly more refining and perfecting his electronic cheat sheet. In his point of view, his work is tied to the sheet itself - he may not fully realize the additional value of the knowledge he gained during the preparation.
I was quite surprised that the student feels so upset
He is upset because from his point of view you destroyed hours of his work. For that it does not matter whether he could transcribe his digital sheet. It does not matter that he would be fine anyway given he is already well prepared. You might as well take a handwritten cheat sheet and literally rip that apart.
So to answer the question yes it is unreasonable to change the rules on short time, particularly with the given reason. And no I do not think the student is overreacting.
I don't like your reason, because I believe students should be given some flexibility to figure out how to learn effectively on their own - and that works differently for every student.
Edit: It seems to need clarification as it is not obvious to everyone. Creating a high quality original cheat sheet is a very time consuming process. Not only do you revisit in detail the entire course topic - which is why it is encouraged in the first place. It can also be very challenging to design the cheat sheet, e.g. compress the information and format it for easy retrieval - especially for a perfectionist.
This is something different than a copy-pasted cheat sheet. From the description it is almost certain, that the student in question did not just copy-paste his cheat sheet. Do not assume others use your own workflow and spend the same amount of time on preparation.