changing a parameter value in a query step

I have some difficulties understanding what you are trying to do and what you mean with a "template".

Anyhow, if you have a parameter defined in Power Query like:

Then you can perfectly have query code (or a function parameter) with the same name "Parameter1" and another value, like:

let
    Source = #table(1,{{1}}),
    Parameter1 = "John",
    Custom1 = Parameter1
in
    Custom1

Actually, when reading multiple files from a folder, various query objects are automatically created, including a parameter that points to the example file and used in the example query, while the corresponding function has a parameter with the same name, overriding the parameter value that points to the example file.