How can I return to non-immigrant status if my I-485 application gets rejected?

Stay with your current H-1B until all actions on your I-485 are completed. Thus, if your I-485 were to be rejected, your current H-1B will remain valid and you can continue on it including, but not limited to, an eventual renewal if necessary.

H-1B visa allows for dual intent. Therefore, if your I-485 is rejected you can indeed fall back to your H-1B visa. Make sure not to change jobs during the period, though, because the H-1B visa, as you must know, is employer-specific. If you stop working for the employer who sponsored your H-1B and your I-485 is rejected, you may be out of luck.

You are fortunate in that the H-1B visa status allows immigrant intent. That means that you may conserve your H-1B status despite applying for residency in the U.S. If the I-485 gets rejected you may continue to stay in H-1B status.

Maintain your H-1B employment terms and do not work on I-485 EAD until your I-485 is adjudicated.

You can still work, travel and maintain H-1B status while I-485 is pending, provided that you have not traveled abroad and re-entered using an advance parole, or you have not violated the conditions of employment under H-1B.

H-1B (and categories like E-2, L-1, O-1, P-1, F-1 and B-1/B-2) are all "non-immigrant" categories, requiring an intent to depart the U.S. when the status is complete. When you file an I-485, you are notifying the government of your "immigrant intent." That is, your intent to remain in the U.S. permanently. Laws and regulations generally require one or the other, but certain categories can allow for "dual intent." H-1B is one of those categories, so you should be able to maintain H-1B status during EB-5 processing. If you fail to maintain H-1B status by working for a different employer, however, that may result in the loss of the dual-intent protection.

If your application for adjustment of status on Form I-485 is denied, your underlying H-1B visa status may still be valid, assuming it has not expired. Given the fact that your EB-5 petition on Form I-526 was approved, it is unlikely that your application for adjustment of status will be denied. You would have the option of working. Once you file your application for adjustment of status, you would have the option of filing contemporaneously therewith for an employment authorization document (EAD), which would take at least 90 to 120 days to be issued. Once issued along with an advance parole (a travel document), should you choose to work after you re-enter the U.S. on an advance parole rather than your H-1B, you would have to work on your EAD. Should your I-485 application be denied, you would be able to depart the U.S. and re-enter in H-1B status. There are some pros and cons, but normally you might wish to work on your EAD to "save" your H-1B time in the unlikely event your I-485 application was denied.

If you think there is any reason why your adjustment should be rejected, you should stay on your H-1B. If the adjustment is denied and you have an H-1B, then you can fall back on the H-1B. Do not present the EAD to an employer and make sure your H-1B is valid.

When you are filing an adjustment of status, you are signaling to USCIS that you no longer want the previously held status. Thus, if the adjudication is made on that I-485, you are no longer holding the previous visa status, as the latest status request is the one you are supposed to have. If the underlying immigrant petition is approved, the only reason that your I-485 will not be approved is because you have an inadmissibility issue, which should have been discussed with your lawyer when you started this process. Because you obviously were given an H-1B visa, up until that visa issuance, you did not have an inadmissibility issue. If you have not disclosed any pertinent issue or have acquired a new issue, I would strongly advise you to consult with your immigration lawyer now and discuss a strategy for dealing with that issue.

As long as you maintain your H-1B, this non-immigrant status would continue if your I-485 is denied.

One can maintain H-1B status during a pending I-485.