An Apple switch is unlikely due to their significant in-house advantages being geared towards ARM directly. PA semi was an ARM shop and their innovation in their A$ line of chips is driven by significant experience working with the ARM ISA... They have a licence which gives them the freedom to design their own chips and unless the new owners decided to do something disastrous with respect to existing contracts, that license will remain valid and its terms unchanged. Something ephemeral to the facts of the existing contracts is unlikely to motivate Apple to do anything so drastic. They already have the ongoing Intel sky lake laptop debacle and a future of less and less certainty regarding Intel's promises to deal with.

Tldr: Apple are more likely to move macOS to ARM than iOS to RISC-V