How can I change my conditional EB-5 green card to a permanent EB-1C green card?

Generally, unless you have a non-immigrant visa you can fall back on, you have to leave the US and wait for the EB-1 immigrant visa. You cannot stay on the EB-5 immigrant visa while waiting for the EB-1. Advisably, the EB-1 which you seem to prefer is fraught with the danger of either being rejected or may even take longer to process. Prior to making a final decision consult an attorney to understand your options.

You will have to surrender your current green card and apply for the new EB-1C green card abroad. A permanent resident cannot adjust except under limited circumstances under Matter of Stockwell.

Only way to do this is to abandon the other green card outside the U.S., but then you will need a nonimmigrant status to return on.

To do this, you would need to abandon your current permanent residency, and seek it through the EB-1 category.

Unfortunately that is not possible. You cannot adjust status if you are already a permanent resident. You could leave the U.S. and file an I-407 to abandon your residency and then try to immigrate based on your approved I-140. Lots of things could go wrong in the process and it would take a very long time.