Sorry, you will be limited to $750,000. The new law says at Section 163(h)(3)(F)(III) that the limit for your new mortgage (post-December 15th, 2017 debt) is $750,000 MINUS the $400,000 (pre-December 15th, 2017 debt).
https://www.law.cornell.edu/uscode/text/26/163#h_3_F
