All guidance I've seen simply requires reporting to be consistent from period to period. That shouldn’t preclude a (very) occasional change in method with good reason.
That said, my question as an auditor would be why you want to introduce unnecessary complication? When I get a new plan audit on the modified cash basis, I am overjoyed to go with that. Accrual is an added hassle to reporting (and incurred time) that, really, no fiduciary I’ve ever met cares about as long as everything is correct. (I’m assuming by “cash” you already mean modified cash, which “utilizes the cash basis of accounting while carrying investments at fair value and recording investment income on the accrual basis. All other transactions are recorded on the cash basis.” True cash without investment valuation changes within a plan is too weird for me to visualize.) Plans can be audited on the modified cash basis.