Calendar year straight PS plan.

Employer went from Sole Proprietor to a professional S Corp effective April 1, 2008

Also added safe harbor 401(k) provision effective May 1, 2008

Plan is to remain a calendar year plan.

Do we need to file two 5500 forms for 2008? One for the sole proprietor, and one for the S Corp because of the change in ID #? Or can we simply use that section on the 5500 form where it asks if there has been any changes to the name and/or EIN of the plan sponsor?

The plan is NOT terminating, it's the same plan, and same sponsor: Only the entity and ID changed as well as adding the 401(k) provision.

On page two of 5500 - line 4 is space for listing Name and EIN changes. For 2008 the corp name will show on line 2a - then you will put prior sponsor and EIN on line 4.

It's not the same sponsor if it's a new entity. Assuming the new document with SH provisions shows the new corp as sponsor, it's usually appropriate to have the sole prop be an adopting employer so the full year is covered by both entities.

But to answer your question, yes, just show the old ID on the 5500 as JanetM explained.