What purposes does the sponsor want the amendment to be retroactively effective for?
If for 401(a)(4) or 410(b), you can do it as a 1.401(a)(4)-11(g) amendment, although you indicated that there was no coverage or nondiscrimination failure that would need correcting.
If for 401(a)(26), you can do it as a 1.401(a)(26)-7(c) amendment, which operates under similar rules to -11(g).
If for minimum funding/maximum deduction purposes, too late. The deadline to adopt a 412(d)(2) amendment for 2021 was March 15, 2022.
If none of the above: then just make the amendment effective in 2022, and give them credit for 2021 for accrual service.