Bitcoin migrating to a ASIC resistant mining algorithm like RandomX used on Monero What would need to happen for Bitcoin to fork to a ASIC resistant mining algorithm like RandomX used by Monero for CPU mining only?
The bitcoin community would have to decide that they don't particularly care about having significant security anymore. ASIC-resistance weakens the security of proof of work chains because it eliminates the need to invest in a chain in order to attack it.

Someone who owns lots of expensive ASICs will not want to let them be used to attack the chain they are mining because if they harm that chain, they will significantly reduce the value of their own ASICs. By contrast, if you can mine with commodity hardware, attacking the chain does not significantly reduce the value of your hardware, so you have no real incentive to prevent your hardware being used to attack the chain. Worse, commodity hardware is easy to rent or steal (with malware) to attack a chain.

So the entire bitcoin community without have to change their opinion on how to make a chain secure to a worse opinion.

It's also important to understand the security is a huge limiting factor on price. If the benefit from causing a double spend exceeds the cost of doing a double spend, there will be double spends. Nobody wants that. So market forces will act to keep the benefit of a double spend below the cost of a double spend.

ASIC-resistant algorithms have much lower cost to do double spends (because you don't need to invest in ASICs but can instead rent or hack commodity hardware) so the profit from a double spend must necessarily be much lower. That will mean either waiting for many more confirmations, dropping the price of bitcoin, or some of each. Who wants that?