How to convert between bases partially?

Question

I have some large number $n$ in base $b_1$ and I want to convert it to base $b_2$. $n$ is about a million decimal digits long, therefore it's impractical to convert it as a whole. However I don't need the entire $n$, just some arbitrary slice of it when it's converted to $b_2$

For example converting from base 10 to base 3, with a slice from digits 5-10:

n(10) = 2347892347
n(3) = 2000[11212]22021210211
m(3) = 11212 <-- the number I need

note: it doesn't matter if the slice starts from the left or the right of the number

Is there a fast method to get that slice $m$? Any help is very much appreciated.