Yes, if you are enrolled in one account and want to participate in the other one for the upcoming plan year, you can choose to switch, as long as you actively elect the other account during Annual Enrollment AND you are also enrolled in either the National Plan or the Whole Health Plan for the upcoming year.
If you switch from an HSA to a PWA, any unused HSA funds will remain in your HSA account. If you switch from a PWA to an HAS, any unused PWA funds will be moved to a Limited PWA, and those funds can only be used for eligible dental and vision expenses.