^^ It appears you can do it without too much trouble. You can submit the financial accounting year, however will need two separate sets of tax computations based on two years of assessment.
http://www.iras.gov.sg/irasHome/page.aspx?id=1388
You have three months following your existing year end to file your estimated chargeable income. You then have until the end of November to file for tax.
Annual meetings are required within 15 months of the last AGM, so if you had it in March typically June is the latest you can go in the subsequent year. Typically my clients will start to question if they need an extension after six months of their year end because of this.
To file for extension I believe it is $50 for the first month, $100 for two. I'm sure they charge more the longer you take.
All that said, to avoid issues, I would notify them within the first six months of your prior year end (of the change in financial year). If your company is a partnership or sole proprietorship you don't need an AGM. So you will likely need to notify them before the November tax filing date in this case.