Are you filing a Family-based Application or an Employment-based Application? If you are already working legally with your company then you can work in any position they feel you are qualified to work. If you are already employed there and have been legally working there, WHERE in the company you work makes no difference in the eyes of the government as long as the salary you are being paid will be the same as or more than you were making before. The reason is, that if you are filing for an adjustment of status based on employment, they need to be sure you will be financially able to support yourself as said in your employer's statement on the I-140 form that was included wth your I-485 application:
-- Employment Letter.
If your adjustment of status application is related to an employment based visa petition (Form I-140), you must submit a letter on the letterhead of the petitioning employer which confirms that the job on which the visa petition is based is still available to you. The letter must also state the salary that will be paid.
So, as long as you will be working for the same company, making at least what you were making before, and have followed all the legal steps in your paperwork, you should be fine to go ahead and apply for, or accept, a promotion.
I hope this has helped, and if you have further questions, please feel free to contact me.