While there is no time limit for requesting reimbursements from an HSA, they must be incurred after the HSA was established (https://www.irs.gov/irb/2004-33_IRB) .
I normally pay for my medical expenses out of pocket, and instead save the receipt. This gives me the option to treat my HSA as an IRA (if it makes it to retirement age) OR as an emergency fund as I have been savings my receipts through the years.
Delaying reimbursements from an HSA and savings receipts to be reimbursed at a future time comes into a problem if you change custodians/employers.
Would I have to request reimbursements for all my receipts prior to changing custodians?
Or can I transfer all my assets to the new custodian and submit receipts to the new custodian from expenses incurred during the previous custodian?