Can a will still be challenged in Ontario after probate has already been granted?

Yes, though it becomes more difficult once a Certificate of Appointment of Estate Trustee has already been issued. A grant of probate does add a layer of formality and reliance that a court will weigh, but it isn't an absolute shield against a later challenge — an interested party who has genuine grounds, such as newly discovered evidence of lack of capacity, undue influence, or the existence of a later will, can still bring an application to have the existing grant revoked and the will's validity properly examined.

Practically, timing matters a great deal. The longer an estate has been administered under the existing grant, and the more the estate trustee, beneficiaries, and third parties have already relied on it — for example, by distributing assets or closing accounts — the more complicated undoing that becomes, even where the underlying concern about the will turns out to be legitimate. Courts will also expect a reasonable explanation for why the challenge wasn't raised earlier, if the grounds existed at the time probate was granted. Anyone who becomes aware of a real problem with a will after probate has already issued should get legal advice as soon as possible, since delay works against them on both the evidence and the practical remedy.