Power Query Custom replacer function

Hi,
Can someone help with information or direct me to post or blog re how the replacer function works in M ,

I have basic replace value ;

Table.ReplaceValue(  Source, "A", "XX", Replacer.ReplaceValue, {"Item"} )

which can be re written ;

Table.ReplaceValue(  Source, "", "", (x, y, z)=> if x = "A" then "XXX" else x , {"Item"} )

but to take a stage further and replace based on another column;

Table.ReplaceValue( Source,
each [Unit] ,
each if [Item] = "A" then [Unit] * 10 else [Unit] , Replacer.ReplaceValue, {"Unit"} )

But I cannot figure how to re write with 'custome replacer' , I have tried various such as ;

=

 Table.ReplaceValue(  Source, 
 each [Unit], "",   (x,y,z)=> if [Item] = "A" then  x * 10 else x ,  {"Unit"} )

Which I thought item = "A" then z = current valuue * 10 ?

Richard.