The Recovery Rebate Credit is only based on your 2020 federal tax return, your filing status and your 2020 AGI.
IRS website for the Recovery Rebate Credit - https://www.irs.gov/newsroom/recovery-rebate-credit
Economic Impact Payments were based on your 2018 or 2019 tax year information. The Recovery Rebate Credit is similar except that the eligibility and the amount are based on 2020 information you include on your 2020 tax return.
I am referring to the new $1400 stimulus check just passed by the Senate not the Recovery Credit.
Since the legislation has not passed and has not been signed by the President nothing has changed in the tax code. When and if the legislation is passed the IRS must write the regulations on how the stimulus payment will be distributed and which federal tax return the stimulus will be based upon. Since nothing in the legislation seems to show this as being retroactive the assumption can only be it will be based on the 2020 federal tax return.