vim: least key stroke to replace a word

experts:
I know this looks stupid and simple, but I don't know the answer right now.

say I want to replace a word :

from:

abc 456

to:
123456

what I did is:
1. put my cursor in a, then cw123<esc>x , 7 strokes
2. put my cursor in b, then caw123<esc>, 7strokes
3. put my cursor in a, then dwi123<esc>, 7 strokes

it looks cw and dw behaves non-consistently, which I know is by the design.

I'm editing an old doc full of typos and spelling errors, etc, and I
need to do this quite often, does anyone knows of a
least-strokes-method (to protect my fingers) ?

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