You may keep 401k plan without any additional contributions - and your former employees may leave their investments - especially if the plan performs well.
You (as an employer) may terminate your plan when it no longer suits your business needs. Typically, the process of terminating a 401k plan includes amending the plan document, distributing all assets, and filing a final Form 5500 - mark "final report" check box.
You must also notify your employees that the 401k plan will be discontinued.
See Form 5310 and the Form 5310 Instructions for additional information.
See some additional requirements here - http://www.irs.gov/retirement/participant/article/0,,id=151798,00.html
Check with your plan’s financial institution or a retirement plan professional to see what further action is necessary to terminate your 401k.
Let me know if you need any help.
Yes - for termination of a 401k plan - the form 5310 is required.
There are few exemptions - they are listed in instruction - but I do not think they affect your situation.
Do to file on your own - please coordinate with the plan administrator.