Sole props deferral election after year end

Under SECURE, sole-prop can adopt a 401k plan after year end and prior to 4/15 (as far as I know, no extension in filing allowed)

How about changing deferrals for an existing plan? Do they need to be done by 12/31 or can be done by 4/15 following?

Thanks

If an individual owns the entire interest in her unincorporated trade or business and is its only employee, an elective deferral under a retroactively established plan is treated as having been made before the end of the plan’s first plan year if the proprietor makes her elective-deferral election before the time for filing her Federal income tax return (without any extension) for her tax year that ends after or with the end of the plan’s first plan year.

This tolerance can apply only to the plan’s FIRST plan year.

Internal Revenue Code of 1986 (26 U.S.C.) § 401(b)(2) http://uscode.house.gov/view.xhtml?req=(title:26 section:401 edition:prelim) OR (granuleid:USC-prelim-title26-section401)&f=treesort&edition=prelim&num=0&jumpTo=true.

This is not advice to anyone.

Peter thank you.