If I send my I-526 form and deposit the 500K in direct EB-5 in October 2021, and the program changes the investment amount to 900K, must I invest the difference to continue the process?
No one knows yet.
It depends on future regulations or rulings to determine if the changes (or reversions) would be retroactive or not.
Likely no. At the very least, if they do that, someone will fight the applicability of such a change in court and you will have time to add to your investment.
Generally, no. The change in the investment amount would need to be made retroactive in order for such to be the case and that is not very common.
Possible, but highly highly unlikely. The general rule is you establish eligibility at the time of filing. Right now it is $500K so unless USCIS wins the "Behring" appeal (which so far seems unlikely) or Congress passes retroactive legislation (possible, but also very unlikely), it will be $500K. In any event, your filing attorney should have already considered both scenarios and created a workaround/hedge to protect you before filing anyways.
DISCLAIMER: the information found on this website is intended to be general information; it is not legal or financial advice. Specific legal or financial advice can only be given by a licensed professional with full knowledge of all the facts and circumstances of your particular situation. You should seek consultation with legal, immigration, and financial experts prior to participating in the EB-5 program. Posting a question on this website does not create an attorney-client relationship. All questions you post will be available to the public: do not include confidential information in your question.