Conditional change to column name? - inconsistent source naming

Final solution --

I removed this clause from the code:

each Text.Contains(_, "ChangeThisSubstring"))),

This is the condition that was causing the failure, since my source data didn't always contain any instances of "ChangeThisSubstring". With the condition removed, this meant that my "#Added Custom" column name replacement table now had a line for every column in my data, not just the columns that needed replacing -- but for other columns, the "new" name was the same as the old name since the Text.Replace rules didn't affect them. 

Then, realizing that {0} was an index for which column to replace, I hardcoded the column I wanted to update using the index for that column:

Rename = Table.RenameColumns(#"Promoted Headers", Record.ToList(Table.ToRecords(#"Added Custom"){8}))

Now, the name for the index is always "changed" to the Text.Replace version every time data is loaded. If a column name is needed, it's changed to the updated version. If it's already the updated version, it just changes to a copy of the original name.