My school's offering a Direct Subsidized Loan; can the offering change?

My graduate school said I've been approved for a direct subsidized loan. Is this a guarantee based on the information they received from FAFSA, or can it change based on my tax doents? Everything I entered into FAFSA reflects my tax doents for last year exactly, but I've never gotten a loan before so I'm unsure if the loan type is a guarantee or not.