MEP to SEP

An employer is part of a Multiple ER plan.  They decide to leave MEP plan and establish there own SEP plan.  Would the SEP plan be considered a "new" plan? 

Additional Questions:

1) If new plan is established with an effective date of 5/1/2020, can the contributions/compensation be combined in the new plan for the entire year?  (assume 12/31 PYE for both)

2)  Does it matter if it is a Safe Harbor plan?

The SEP is a new plan with a new effective date. All compliance testing be performed for the 12 months.  

Issues I run into is retrieving the data required in a good format when the plan was under a separate provider 

Safe harbor has to be continued unless revoked through the normal process

Ok...New added twist to this question...

MEP adopter terminates services with MEP effective 1/1/2019.  Per agreement, once termination occurs, they immediately are no longer eligible to contribute to MEP.  New plan is not effective until 5/1/2020. 

Can the employer withhold contributions from 1/1/-5/1/2019?  I am assuming no since there was not a "plan" in place.

If the answer is no, if the employer withheld contributions, what should the ER do with them?

Since there is a gap in the time from leaving the MEP to the new SEP, how does this impact testing?  Would HCE determination be from 5/1/18-4/30/19?  What about prorating the compensation limit?  For top heavy, would the 12/31/19 balance be used for 19 & 20?

Let's say, the ER left the MEP effective 4/30/19 and new plan established 5/1/19, based on the above, you would test everything for the full year?