变
biàn
换 swaps the slot; 变 changes the thing itself
The picture The thing stays where it is but turns into something else — what was there is now a different thing.
Where the picture shows up in 变
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the weather turns into something else
天气变了，多穿点。
tiānqì biàn le, duō chuān diǎn
The weather's turned — dress warmer.
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a person becomes another person
几年不见，他变了很多。
jǐ nián bú jiàn, tā biàn le hěn duō
It's been a few years — you've changed a lot.
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become a different substance
水变成了冰。
shuǐ biàn chéng le bīng
The water turned to ice.
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turn it into something else
我们把仓库变成了教室。
wǒmen bǎ cāngkù biàn chéng le jiàoshì
We turned the warehouse into a classroom.
With 换 you'd remove the warehouse and put a classroom in its place; with 变 the warehouse itself became one.
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the plan becomes a different plan
计划变了，明天不去了。
jìhuà biàn le, míngtiān bú qù le
The plan changed — we're not going tomorrow.