Form 5500 - employer transition from non-ERISA to ERISA - short plan year?

Employer was a governmental entity exempt from ERISA until 7/1/23 when it's structure was changed to a non-governmental, non-profit entity.  The employer has sponsored various welfare and retirement plans for years but we were not filing 5500s due to the governmental status of the employer.  Now that the employer is subject to ERISA we are working on the 2023 Form 5500s (yes - we know they are late😊).  For the 2023 Form 5500s, do we need to indicate a short plan year from 7/1/23 to 12/31/23?  By checking the box for a short plan year are we saying the plans did not come into existence until 7/1/23 or that the plans were not subject to ERISA reporting requirements until 7/1/23?   Any guidance is appreciated! 

I have not seen an explicit answer for this situation.

Consider the Form 5500 instructions for Line B:  "Line B – Box for First Return/Report. Check this box if an annual return/report has not been previously filed for this plan or DFE. For the purpose of completing this box, the Form 5500-EZ is not considered an annual return/report."

The comment that a Form 5500-EZ is not considered an annual return/report provides an example of a where a plan existed before the time before this filing, but the box is checked for first return/report.  (We could conclude from this instruction that a Form 5500-EZ is not a type of a Form 5500, but a Form 5500-SF is a type of Form 5500.) 

I expect this ostensibly is because plan that files a Form 5500-EZ is not an ERISA plan.

Assuming that the this box is checked, it would seem reasonable to file the form with an initial short plan year and report all of the participant and financial information for that short plan year.

If someone does have an explicit answer, hopefully they will share it with us.