$750k limit for loan interest deduction

If you refinanced after Dec 15, 2017 but your original loan was before that date, are you grandfathered into the old $1m mortgage interest deduction?  Someone said only new loans taken out after this date is limited to $750k limit for mortgage interest deduction but if original indebtedness is before this date, you are grandfathered in at the $1m rule.  Even if you refinanced.

Yes, it's based on the date of the original debt.

Does it matter if you refinanced with the same bank as original loan or not?  How do they know date of original indebtedness?

No, bank doesn't matter.  IRS system can easily match up the address on the 1098 form.