Q - screw-in block core plug removal

[JonE]

OK. older style coreplugs. A block which I feel like fiddling with to learn how to do things I haven't attempted.
What is the best way to remove the external core plugs with the slot in them... assuming they have been there a long time. Someone mentioned replacing with brass ones?
This block also has the little ?brazed/soldered extra plugged core holes at the ends of the valve area. 
Are these something worth disturbing for a clean up inside - or leave well alone?