Form 5500SF and Code 4R

We have a small employer health plan that is maintained in conjunction with a trust.  Therefore, it is funded and we file a Form 5500SF.  We have assigned the filing Plan # 501.  When the plan terminates the trust and switches over to a fully insured product we list Code 4R saying the plan will not file a Form 5500SF going forward.  We believe this is better than indicating it is a final filing.  In other words, the plan continues, but the funding method has changed; but the plan is not terminated.  If we marked the filing as final, then we would have to use Plan # 502.  

Does anybody have any thoughts on this?  Clearly we have to use Code 4R or mark it as final; otherwise the government will expect another filing.  Again, we think the better approach is to use Code 4R rather than indicate it is a final filing when the employer simply continue the plan but changes the funding method.  Thanks.   

Use Code 4R. The plan has not terminated. Only the funding method has changed. Huge distinction. 

For what it's worth, I practiced employee benefits law for 15 years.