Company merger, move to new HDHP and HSA mid-year. Can I use funds from new HSA acct for exp claimed against the old insurance or should I transfer funds to old HSA?

Been in a HDHP/HSA for several years.   The move mid-year to another HDHP/HSA is due to my employer changing carriers/administrators post-merger.  I still have some remaining 2018 expenses and will have 2019 expenses prior to the switch for which funds will be payroll deducted into the new HSA.