How to replace searched lines with yanked buffer?
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If I have a file

    aaa / new replacement line
    AAA / old target line
    random line
    random line 
    BBB / old target line
    CCC / old target line

 
In this example there are 4 replacements,
so the final file should end up like


    aaa / new replacement line
    aaa / new replacement line
    random line
    random line 
    aaa / new replacement line
    aaa / new replacement line

I have an attempt at an answer below but would like to see your input on this also.
Maybe there is a more successful 'vim golf' approach I am unaware of. Maybe I'm just being really dumb and there is a simple command for this.

Basically, the question is if I have a line yanked into a register, I want to completely replace searched lines matching 'old target line'.

