JavaScript (Node.js), 95 bytes

x=>x.toLowerCase(a=/[a-z]/gi).replace(/_/g,g=i=>i>62?(i-52).toString(36):g(a.exec(x)<{}|2*-~i))

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JavaScript (Node.js), 125 124 bytes

x=>x.replace(s=/./g,c=>parseInt(c,36)>9?c.toLowerCase(s+=+(c<{})):c=='_'?('0b'+s.slice(4,9)-~9).toString(36,s=s.slice(5)):c)

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JavaScript (Node.js), 94 bytes

x=>x.toLowerCase(w=/[a-z]/gi).replace(/_/g,g=Z=>Z>62?(Z-52).toString(36):g(w.exec(x)<g|2*-~Z))

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JavaScript (Node.js), 125 124 bytes

x=>x.replace(s=/./g,c=>parseInt(c,36)>9?c.toLowerCase(s+=+(c<{})):c=='_'?('0b'+s.slice(4,9)-~9).toString(36,s=s.slice(5)):c)

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JavaScript (Node.js), 125 124 bytes

x=>x.replace(s=/./g,c=>parseInt(c,36)>9?c.toLowerCase(s+=+(c<{})):c=='_'?('0b'+s.slice(4,9)-~9).toString(36,s=s.slice(5)):c)

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JavaScript (Node.js), 95 bytes

x=>x.toLowerCase(a=/[a-z]/gi).replace(/_/g,g=i=>i>62?(i-52).toString(36):g(a.exec(x)<{}|2*-~i))

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JavaScript (Node.js), 125 124 bytes

x=>x.replace(s=/./g,c=>parseInt(c,36)>9?c.toLowerCase(s+=+(c<{})):c=='_'?('0b'+s.slice(4,9)-~9).toString(36,s=s.slice(5)):c)

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