Profit Sharing Plan Adding 401(k) Features

A PSP that was set up many years ago is now being amended into a 401(k) plan.  Once this occurs, does it have to include LTPT employees or is it excluded from this requirement by virtue of the fact that the plan has existed for a long time?

LTPT doesn't have the same grandfathering rule as the auto-enrollment.

401(k) Plans, existing, new or converted have to comply with the LTPT rules. 

Yup, I got it confused with the autoenrollment rules - thank you both.