I submitted my abstract (400 words) to a conference, and unexpectedly it was accepted to the poster session, not the oral one that I wish. The submission of the full paper is not mandatory for that conference. Honestly, going to that conference costs me a lot in terms of time and money, so if I have to present a poster, I would like to withdraw it. My supervisor suggested me to instead submit the full paper to a journal because I already have the full paper. I just want to make sure if it would be okay. Any advice will be appreciated!

I think that should be fine. Submitting to a conference is not the same as submitting to a journal, so I don't think there should be any conflict or academic ethic issues. Many papers that are submitted to conferences are later revised based on feedback and submitted to journals, especially if there are no conference proceedings, or if conference proceedings are no longer being considered proper publications.

Submitting the paper to a journal is better for your prospects anyways, and you can pull out of the conference if you are not planning on attending.

I certainly don't consider that when I submit an abstract for an oral presentation, that accepting it as a poster places any obligation on me! Some conferences ask you if you'd like to be considered for poster presentation, others assume. I agree that for the expense involved, it may well not be worth it.

you can certainly submit to both a journal and conference, no harm in that