How long must a plan be a safe harbor plan to avoid testing?

We have a client who gives their employees a phenomenal match, but are still failing their testing. When during a plan year can it be switched to a Safe Harbor Plan, if at all?

A plan which already is a 401(k) plan can switch to safe harbor only as of the beginning of a plan year.

I you wanted, you could change the plan year and move to SH starting with the plan year immediately after the short year.