Other than a conversion to a cash balance plan, are there any other practical means for converting a unit benefit DB plan to a DC plan?

I'm not quite sure what you mean by "conversion." If you mean as in converting a Money Purchase plan to a Profit Sharing plan, I would say no. See ERISA 4041(e).

You cannot convert a defined benefit plan to a profit sharing/dc plan. You must do a termination of the DB plan.

The problem is protection of benefits in DC plan where account values can decline.

When you say "other than a conversion to a cash balance plan", you are making a misinformed statement. A cash balance plan is still a DB plan. It is not a DC plan. It simply has some similar features.