Hey guys, intermediate DM here with a problem/question.

One of my players started the campaign as a monk, and when he got to level 3, he chose Way of the Four Elements for his subclass. He’s now about to hit level 6 now however, and he’s approached me about changing his subclass. Reasoning:
1. He doesn’t enjoy WotFE. 
2. He has recently bonded very closely with the ancient gold dragon that is sorta the party’s patron, and feels like Way of The Ascendant Dragon makes more sense for his character.

So my question is: Have any of you guys ever made a house rule on changing subclasses? If not, any suggestions on how to go about it making sense? Also, should I even pursue this avenue, or just tell the monk that he needs to work with what he’s chosen?

I would just let it happen. WotFE is not a well designed subclass so his grievance is understandable and he's already got a way for it to work in the story. Just let him do some ritual with the dragon or something and switch out the character sheets.

In general, you can't make someone play a character they don't want to play anymore. They either get resentful and try to get their character killed, or they get apathetic and stop enjoying the game. You shouldn't be swapping out characters all the time on the slightest whim, but if you can tell that someone's dissatisfaction is genuine and not likely to change then figuring out a way to change it is best for the game.

I have not made such a houserule, but I would. The whole point of D&D is to have fun. If the player isn’t having fun, why should he play? Make sense? My first inclination is to handle it by means of a side quest. Have the dragon offer to officially patronize the monk, and by extension the party, if they do a relatively short quest for it, or pass a brief series of tests to prove their worthiness. Then, when the heroes succeed (because that’s the point), then bingo bango, the monk gets a new groove and the party gets a group patron.

Tasha's Cauldron of Everything actually suggests allowing a character to change their subclass at a level where they would normally gain a new subclass feature. Monks gain subclass features at level 6, so it's a perfect time for them to change!

The Tasha's Cauldron suggestion can be a bit restrictive though, as some classes have sub-class features quite far apart. Personally I think the only house-rule required is "explain it".

I'm personally a fan of dragons that can mark those who have gained their favour, so you could for example have the character receiving visions as they sleep/meditate during a long rest, and awaken to find a burning tattoo emblazoning itself upon their hand/arm/chest, marking the change.

You could take it a step further and ask them to RP the character as unfamiliar with their new abilities; maybe they try to use a four elements power they once had, and it manifests as an ascendant dragon ability instead? Maybe a fight pressures them into discovering how a new power works? Or they spend part of the long rest having to experiment and learn what they can now do?

And just generally; try to make it cool. It's a character moment, and they should be embellished and celebrated as much as possible. 😄

Thanks for the suggestions guys!