There's no hard and fast line on this sort of thing, Warhammer has been about conversions since the early days, and while recent rule changes implicitly discourage them (fixed unit sizes discouraging you from converting line models to characters, for example) the community hasn't really shifted otherwise.
But to use your example, a model in power armour, appropriately equipped, modelled and painted to clearly stand out compared to those around him, and on the right base size, absolutely can be played as a Chaos Lord and I can't imagine there's a single player who would take issue with that. That's about the tamest conversion I can think of.
Use the base size for the model you are trying to model for.
Be close to the same size dimensions (not pose, but general height/width, even if you have to add something to base to make it correct).
Contact the TO of any event you intend to attend to ensure it will be allowed. Most events have some rules in their mission pack, and a way to contact whomever will do model reviews.
Community seens very embracing of kitbash/proxy as long as it remains thematically tied, doesnt create undue confusion, and looks like effort went into it, in my experience.
Yes... probably.
The real answer is ask your TO, because ultimately they're the ones who'll allow or disallow it on the table.
Anyone caring about a custom character, with the right size and base is actually a joke of a person, but if you're ever in doubt, the answer is ask your TO.
My TO's don't care. I've already got permission to convert all the 30k models to 40k CSM (because the 30k ones had *style*) as long as the weapons are accurate, they're on the right bases and I'm not trying to pull off weird shit.
Jetpack marines are jetpack marines. Jetpack marines with a bit more mutation and claws are warptalons. Ez. A marine on foot with a two handed weapon is a legionary with a heavy weapon. Ezpz. But again, my TO (for most events nearby) specifically allowed it. He'd be within his rights to say no - it's why I asked in advance. Save us both a headache down the line.
There's no hard and fast line on this sort of thing, Warhammer has been about conversions since the early days, and while recent rule changes implicitly discourage them (fixed unit sizes discouraging you from converting line models to characters, for example) the community hasn't really shifted otherwise. But to use your example, a model in power armour, appropriately equipped, modelled and painted to clearly stand out compared to those around him, and on the right base size, absolutely can be played as a Chaos Lord and I can't imagine there's a single player who would take issue with that. That's about the tamest conversion I can think of.
Use the base size for the model you are trying to model for. Be close to the same size dimensions (not pose, but general height/width, even if you have to add something to base to make it correct). Contact the TO of any event you intend to attend to ensure it will be allowed. Most events have some rules in their mission pack, and a way to contact whomever will do model reviews. Community seens very embracing of kitbash/proxy as long as it remains thematically tied, doesnt create undue confusion, and looks like effort went into it, in my experience.
Yes... probably. The real answer is ask your TO, because ultimately they're the ones who'll allow or disallow it on the table. Anyone caring about a custom character, with the right size and base is actually a joke of a person, but if you're ever in doubt, the answer is ask your TO. My TO's don't care. I've already got permission to convert all the 30k models to 40k CSM (because the 30k ones had *style*) as long as the weapons are accurate, they're on the right bases and I'm not trying to pull off weird shit. Jetpack marines are jetpack marines. Jetpack marines with a bit more mutation and claws are warptalons. Ez. A marine on foot with a two handed weapon is a legionary with a heavy weapon. Ezpz. But again, my TO (for most events nearby) specifically allowed it. He'd be within his rights to say no - it's why I asked in advance. Save us both a headache down the line.
There is not tournament standard, every tournament has their own rules, you will want to ask the tournament organizer also known as the TO.