Potential dream job when I have already accepted another

[ZednotZee]

I have accepted a position and have signed a contract, due to start with the company next month.
I have spotted a dream role, better pay, better benefits and more seniority and have had a preliminary interview which went really well.
If I am offered this job what are the consequences of declining the role which I have already accepted?

I know its bad form but these jobs almost never come available and I don't want to miss an opportunity should I be offered it.

TIA.

Well they won’t be happy but if they needed to get rid of you before you actually started or in the first week they would.
You may technically need to work your notice at the new job but it’s unlikely they would make you if you hadn’t actually started.
Do what’s best for you while being as polite and professional as possible