How is the best way to purge brake lines

I have to do a brake job on old Betsy and need to totally change the
brake fuid at the same time.

Is it best to totally drain the system, do repairs and then re-fill
eveything?

Or just undo what I have to and block the flow and then fill and bleed
until it is all done?

Or does it make any difference?

If it will not affect or cause issues, number 1 would be best I think
as I have to replace brake caliper seals in all 4 wheels. And I do
want to totally change the fluid as it has been years since it was
done - like 20 or so....

TIA...