How can I switch from a visa exempt to an ED visa for studying muay thai in Thailand?

You mean 180 day given to you by your Thai girlfriend?

You hear stories from people saying that the immigration officers on arrival warn of this limit (and therefore take it as gospel), but there is no rule saying you can only stay 180 days.

There's no 180 day limit

There is no 180 day limit. It's up to the discretion of immigration at the time of entry. But if you've been in Thailand that long you're correct that you're probably going to be denied entry soon as you cannot live in Thailand on short term stays.

To get an ED you need to contact schools and find out their process. Some will be able to convert from visa exempt or tourist visa to ED without leaving Thailand and some will want you to leave and apply for an ED visa at a consulate.

You'll need 14-21 days on your stay to convert though so your cannot wait until you're out of time to try and do so.

You can utilize an ED visa and extensions for up to 1 year if it's not a university degree course. Your attendance will be recorded and repoerted to immigration when you go and apply for your extensions every 90 days and if you're not attending you'll be denied.

180 day limit?