I'm in the same place as you.. Doing science but wanna do commerce after.

I like the idea of studying science before going into commerce. But I do have the same questions sometimes. But as a result, I will be completing the subjects for an Informatics major so that I can move into some i.t/business fields hopefully after my undergrad finishes. Then from there with at least 2 years work experience, return for a masters in marketing or management.
And i'm pretty sure even if you study science, you can still move into business masters. Melbourne Business School has no prereqs, but look at your gpa, extra curricular and work experience, if i'm not mistaken.
At the same time, Melbourne Business and economics have a Masters in Management where you can specialise in accounting, marketing, etc.. And there aren't prereqs for these again, just gpa, etc. So you do have options after your science degree for further study if you wanted.

For me, the only downside of doing science before going into commerce is the many more contact hours D: ... So lazy.