401k to Simple 401k

Is a Simple 401(k) considered a successor plan to a 401(k) plan. 

We have a client who wants to terminate their 401(k) plan as of 12/3/19 and start a simple 401(k) plan as of 1/1/20.  Is this allowable under the successor rules?

no need to terminate, just convert the plan

for instance the ERISA Outline Book points out (emphasis mine)

Chapter 11  section XIII part D

Part D., Plan year must be the calendar year

Rev. Proc. 97-9, section 2.04, clarifies that the plan year must be the calendar year if the SIMPLE-401(k) provisions are adopted. Also see Treas. Reg. §1.401(k)-4(g) (December 29, 2004). The IRS has taken this position because SIMPLE-IRA plans under §408(p) must define the plan year to be the calendar year, and IRC §401(k)(11)(D) incorporates the IRC §408(p) definitions by reference. If an existing 401(k) plan has a fiscal plan year, the plan year must be amended to the calendar year if the plan is being converted into a SIMPLE-401(k) plan. See Form 5308 for instructions on whether the amendment of the plan year is eligible for automatic approval or whether Form 5308 must be filed for IRS approval on the amendment

What does the 5500 look like for the plan's last year as a 401(k)?

never mind - Simple, not SIMPLE.