If plan sponsor changes name or type of entity they operate under from s corp to llc for ex;is amendment needed?

If the document is unamended, will it inaccurately reflect information about the sponsor?

Generally, yes. I might worry less about a pure change of name, since it's really the same sponsor, but I would eventually change it. For a change of entity, you really need an amendment because...it's a new entity/sponsor.

Certainly amending the plan to change the name & the entity type, with nothing else, might not be sufficient (unless the plan already contains definitions and provisions--like earned income--that will allow the plan to work equally well for corps & self-employed entities without amendment).

In any event, you should amend the plan to reflect the change in info required to be provided via the SPD (name of employer/sponsor and, perhaps, EIN). So, at a minimum you'd have to provide an SMM with this new info at some point.