Can a grant of probate already issued be set aside later if a problem with the will is found?

Yes. A Certificate of Appointment of Estate Trustee already issued by an Ontario court isn't necessarily the final word if a genuine problem with the underlying will later comes to light — for example, if a later, valid will is discovered, if serious evidence of lack of capacity or undue influence emerges after the fact, or if the certificate was obtained based on incomplete or inaccurate information. An interested party can apply to the court to have the certificate revoked or set aside and, where appropriate, have a new certificate issued based on the correct will or a proper administration.

This isn't a routine or easy step, though. Courts are generally reluctant to unsettle a grant that beneficiaries, creditors, and others may already be relying on, so the person seeking to set it aside needs solid, credible evidence supporting a real problem, not just a late change of mind about the distribution. The longer the estate has been administered based on the original grant, and the more third parties have relied on it, the more significant the practical complications tend to be. Anyone who discovers new information suggesting an existing grant of probate may be wrong should raise it with a lawyer promptly, since delay can affect both the evidence and the practical remedy available.