How does changing business structure of a plan sponsor effect a 401k plan

If we have a company that is currently a LLC taxed as a S corp but they are changing their structure to be a LLC taxed as a C corp is there anything specific that we need to keep an eye out for in their plan? I assume we need to make an amendment to update the tax structure and the EIN for the plan sponsor. Outside of that is there anything in the plan design that needs to be updated if a tax structure is changed? They don't want to change anything else in their plan such as vesting, eligibility, ect. 

I am not sure if the EIN changes when the entity just changes the tax election?  If EIN is the same you probably do not even need to amend and you will address the different corporation box (C vs S) at next restatement.

Agree with truphao. If the entity is the same with the same EIN just making a different election on their tax filing I doubt you have any changes other then updating the entry type in the adoption agreement which depending on your provider may or may not require an amendment.

If it is a new EIN, it will be a new entity and you'll want to amend to make the new entity the sponsor.

But I think just switching from S-corp to C-corp or the otherway around, you may find that nothing needs to be done.

If they change from a Sole-prop or partnership to Corporation, in that case you will have a new entity and will need some amendments to have the right plan sponsor.

The IRS says an entity that is an LLC can be a corporation, does not distinguish the LLC as C or S, and does not need to get a new EIN. 

https://www.irs.gov/businesses/small-businesses-self-employed/do-you-need-a-new-ein

If, for some reason, the business does get a new EIN, I agree with the comments above that you will want to amend the plan sponsor keeping in mind that the EIN/Plan Number pair is the unique identifier for plan purposes.