On the 20th of this month I start a new job in a 20 hour per week position in a store that will open its doors next month. Almost a month ago they hired many people at once after a day of group interviews with more than 100 people, including me. I haven't signed the contract yet but I have the pre-agreement. The place and the company are good, so I would like to stay there. My doubt is that this week my availability has changed because I have left school: instead of part-time, I can now do full-time. I want to communicate this to the company because I am interested in working more hours and the idea arises that if full-time vacancies appear before the 20th (because a lot of time passes from the pre-agreement to the actual beginning of the contract, and I think that perhaps someone hired changes his mind), they could count on me.
However, I am afraid that the company will interpret this as something negative and that they will come to the conclusion that they should not hire me because if they cannot offer me more hours of work I will look for another job and I will not be with them for too long. This doesn't have to be this way, I'm willing to wait for vacancies to arise. Furthermore, the company already told me that initially it would be 20 hours a week but that if there is a lot of work they could be extended.
My question is: should I ask them to consider me for full-time positions now, or should I wait to tell them? Money is not an urgency right now and I value above all what will be better in the long term.
Thank you!