Historical Rules · 7 August 2020
Rule 701.28f
Official source text from the 7 August 2020 Rules release
If an activated or triggered ability of a permanent that isn’t a delayed triggered ability of that permanent tries to transform it, the permanent transforms only if it hasn’t transformed since the ability was put onto the stack. If a delayed triggered ability of a permanent tries to transform that permanent, the permanent transforms only if it hasn’t transformed since that delayed triggered ability was created. In either case, if the permanent has already transformed, the instruction to transform is ignored.