There are many changes, including:
The Digital SAT is shorter than the paper SAT. The paper SAT used to take 3 hours and 15 minutes to complete, and the DSAT will take 2 hours and 14 minutes to complete. This means time management is even more critical than it was before.
There are shorter reading passages and only one question per passage on the Digital SAT. On the paper SAT, there used to be multiple questions related to one long passage.
The DSAT is adaptive. Depending on how you perform on the first Reading & Writing and Math modules, your second modules for each section may be easier or harder.
Unlike the paper SAT, on the digital version, you will have access to a calculator for the entire math section. You can practice using the Desmos online graphing calculator here.