The requirements for D1 are listed in Article 20.9 of the D1 manual, and 20.10 of the D2 manual. There are multiple ways of meeting the requirements, but in general, D1 requires a larger athletic program (number of sports offered, number of grants in aid, total dollars, etc.) than a D2 school. For example, if total dollars are used to meet the financial requirement, D1 wants close to 1.1 million dollars, while D2 wants 250 thousand dollars spent on grants-in aid.
The process for transitioning to a different division are also listed in Article 20.