My priority date is July 2015 and I have already filed DS-260 in China last year, what happens if I submit I-485 in US now? is it feasible since the case has to be transferred from NVC process to I-485 process? What are my options?
Yes - you can have both processes pending at the same time. If you are in the U.S. in valid nonimmigrant status and your priority date is current, you can file for adjustment of status. If your case comes up for a visa interview abroad, you can depart the U.S. and pursue the interview process in Guangzhou.
You can file I-485 if you are eligible to adjust status in the U.S. You should consult with an immigration attorney to assess your eligibility. You can withdraw your case once your I-485 is approved.
