Question: endless substitution

I'm begginer in maple so I have many questions

I have 

f1 := [x2^3-3*x1+x2, -x2-x3+u, x2-2*x3]

I want to put 

x1=2*x1

x2=2*x2

x3=5*x3

 

and I wand it in loop

for x4,...xn

so that f1 will be a vector

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