I would modify this answer a little...it is correct that you would need to provide any receipts or documentation to support the adjustment you are requesting CRA to make on your tax return. However, if the change you are requesting increases the amount of a credit or deduction already claimed, you would need to provide ALL receipts and documentation to support your new value. For example...you have already claimed $1000 in RRSP deductions for your 2012 tax return, and you have found/located a receipt for an additional $500. When you are asked to provide documentation, you would need to have receipts/documentation for the whole $1500.
Something else to consider...there are some changes where CRA may already have the documentation. An example of this is when the Disability Tax Credit is approved by CRA, it may often be approved for prior tax years. If this is the case, then you would just need to ask CRA to adjust the appropriate tax years to include the Disability Tax Credit as per their own records.
