When we look at §163(f)(5)(F), it is clear that the grandfathered debt (refinanced amount of the old debt) still gets the $1,000,000 limit. It is just 'new' money that is subject to the lower limit.
(iii) Treatment of refinancings of indebtedness
In the case of any indebtedness which is incurred to refinance indebtedness, such refinanced indebtedness shall be treated for purposes of clause (i)(III) as incurred on the date that the original indebtedness was incurred to the extent the amount of the indebtedness resulting from such refinancing does not exceed the amount of the refinanced indebtedness.
And the "clause (i)(III)" basically says the $750,000 limit does not apply to loans on or before December 15, 2017 (the $1,000,000 limit applies).