CRA considers that the division of an estate into testamentary trusts can accelerate (perhaps to Day 1) the demise of the estate as a GRE

If the will of the deceased provides for the division of the residue into testamentary trusts (e.g., a spousal and children’s trust), the estate can no longer qualify as a graduated rate estate (i.e., even before the passage of 36 months from death) if all the assets become held in those testamentary trusts. Once this occurs, the problem cannot be solved by transferring the assets in the testamentary trusts back to the estate. CRA also considers that the testamentary trusts arise at the time of death.

Neal Armstrong. Summary of 10 June 2016 STEP Roundtable, Q. 1 under s. 248(1) – graduated rate estate.