The decision to make a change from using a cash basis to using an accrual basis for 5500 purposes (assuming that such a decision can be made without governmental acceptance) is, clearly, one that belongs to the sponsor. How could the service provider think the decision was theirs to make?
If past practice was to report on a cash basis (recognizing that Schedule SB reporting is never on a cash basis), there would be no compelling reason to switch just so the 5500-SF (or Schedule H if a larger plan) would match the Schedule SB. Note that if there are receivable contributions, the market value on the Schedule SB will NEVER match the 5500-SF or Schedule H beginning balance, since the former would include the receivable contribution on a discounted basis but the latter would include it on an unadjusted basis. True, it used to be expected that they would match, but since PPA became effective, the SB must reflect for all purposes a discounted value for any receivable contributions.