From my reading of the actual bill so far, cars bought during the transition period (defined as the beginning on the date Tesla was no longer able to get the credit until the enactment of this bill) will count towards the 400,000 additional cars and will receive a tax credit of 7000. I am not a lawyer, but here is an excerpt of Section 401 of this bill
Text of H.R. 848: GREEN Act of 2021 (Introduced version) - GovTrack.us
(1)
In general
In the case of any new qualified plug-in electric drive motor vehicle sold after the date of the enactment of the GREEN Act of 2021—
(A)
if such vehicle is sold during the transition period, the amount determined under subsection (b)(2) shall be reduced by $500, and
(B)
if such vehicle is sold during the phaseout period, only the applicable percentage of the credit otherwise allowable under subsection (a) shall be allowed.
(2)
Transition period
For purposes of this subsection, the transition period is the period subsequent to the first date on which the number of new qualified plug-in electric drive motor vehicles manufactured by the manufacturer of the vehicle referred to in paragraph (1) sold for use in the United States after December 31, 2009, is at least 200,000.