As a chartered accountant in Victoria, this is one of the most common questions that I hear. The simple answer is yes, tax returns may be amended up to 10 years after they were filed. This may be done for personal or corporate tax returns.
Why would you want to amend a return? Sometimes it is to report income that was omitted initially, a common circumstance being receiving a late T3 slip or an amended T-slip from your investment manager or employer. You may also be in the happier position of claiming a missed credit or write-off that was omitted with the original tax return filing. For personal returns this can often mean new credits that you may have been unaware of, such as the home renovation tax credit or certain moving expenses related to moving to your new community. Corporations may have omitted claiming significant credits such as the Apprenticeship training tax credit which can shave potentially thousands of dollars from your return if you employ qualifying apprentices in your business.
Whatever your reason, an adjustment is a straightforward application to the CRA on specifed forms for personal taxes (T1adj) or by letter for corporate amendments to change your original filing. It is not necessary to re-file your entire return.