This is a question and answer in the World of Warcraft Public Test Realm FAQ:

Who can play on the test realm?

Players who create an active World of Warcraft account before the test realms go live are eligible to play on the test realms. There may be other criteria as well, which will be announced with each round of testing if they exist.

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So , this means - If your account cancels one day after a patch goes live, and there isn't another patch for 1 year - you'd still be eligible to play on the PTR for that next patch testing stage?

Am I understanding this correctly?