Simple 401(k) to Standard 401(k)

I have a new client coming on that has been doing a simple 401(k) but are now transitioning to a full 401(k) Plan.  My question is for startup purposes, when I'm filing a Form 5500 would the start date be now (when it's transitioning to a traditional 401(k)) or would it be the date the Simple 401(k) started?

Thanks in advance!