Form 5500-EZ is the filing requirement for Solo 401(k)s with $250,000 or more in plan assets. It is simply an informational return that is filed with the IRS. Solo 401(k)s with assets totaling less than $250,000 have no filing requirement at all.
For additional information on the 5500-EZ form, see the IRS instructions here: http://www.irs.gov/pub/irs-pdf/i5500ez.pdf
You must file a 5500-EZ if you are terminating your plan
If you're rolling the assets over to another 401k provider you shouldn't need to file a 5500-EZ if you are keeping the same trust and tax ID number. However, it is recommended you check with your new document provider.
If you're closing the 401k completely and distributing the assets, or rolling everything into a self-directed IRA or other qualified retirement vehicle, then you must file and submit form 5500-EZ.