The only restriction on using HSA funds to pay for qualified medical expenses for you, your spouse and your dependents is that the expense has to have been incurred after the establishment of the HSA and expenses cannot be reimbursed by some other means. In other words, you can't cover the same portion of an expense twice, once from the FSA and again from the HSA. (Nothing prevents you from paying the expense half from the FSA and half from the HSA, you would just need to keep real good records to show that the expense was not covered twice.)