I'm currently enrolled in a HDHP plan and will be changing job on 9/16. If I do not elect a HDHP plan with my new employer, will I lose all HDHP tax credit?

[dmertz]

Are you asking about HSA contributions or are you asking about Premium Tax Credits?

If you are not covered by an HDHP plan on December 1, 2019, your HSA contribution limit for 2019 will be determined by the number of months that you were covered on the first of the month.  If you are covered by an HDHP for only January through September, your HSA contribution limit will be 3/4 of the annual limit.

Eligible HSA contributions reduce taxable income.  They do not provide a tax credit.

If you are asking about Premium Tax Credits under the Affordable Care Act, you don't get PTC for months that you don't have qualifying coverage under the ACA.