Changing the function within the same company mostly depends on the relationships you have in the company with the people who make decisions in that regard, and the company's policies and culture. You basically need to ask the head of the Finance department if s/he would take you in if you had an MBA. If they really like you and hold you in high regard and would like you to work there, then probably it would not matter if you get full-time or part-time MBA. All they would need to do is mark the check box for your qualification. If the person in charge, culture, or policies do not favor such transitions, then no matter what credentials you have it would not help you.
On a side note, it is generally quite difficult to make transition from science/technical/support function to business function within the same company. I would not call it a stigma, but there is certain perception of you based on your current position/function. It is more common to get promoted with your MBA to a higher responsibilities position if you are already performing similar business-related job/function. That's why many career-switchers look for a job with a new company after getting their MBA. And yes, it is more common for career-switchers to go full-time, even though among my part-time classmates I know at least one person who managed to switch from the military career (program management) to corporate finance.
