The easiest way I know is to convert the base 8 number to base 10, then convert the base 10 number to base 7.

where the 469 is in base 10.

Now to convert 469 to base 7. The largest power of 7 that divides 469 (without being larger than it) is 3:

The next largest power of 7 is 2:

The next largest power of 7 is 1:

And the last power of 7 is 0:

So

I would not be surprised to find that the Euclidean Algorithm provides a way to do this directly from base 8 to base 7, but I don't know how to set it up.

-Dan