Update multiple *.configs

I have a case where I have multiple configs of varying types in many projects within the one solution - e.g. web.config; app.config; logging.config and so on.

I have a need to update a number of appSettings that fit with the CI environment so that unit/integration tests will use appropriate values. Up to now I have used the File Content Replacer but I am finding it increasingly brittle if, say, a subtle change is made to the source xml fragment e.g. changing this <add key="someKey" value="someValue"/> to this: <add key="someKey" value="someValue" />.

e.g. 

Replace: <add key="someKey" value="Existing value with no trailing space"/>
With this: <add key="someKey" value="[Always fails]>

Is there an alternative to the File Content Replacer, something like update [value] where [key] name matches xyz?

Failing that, is there a better way I could be using the File Content Replacer?

Currently running TeamCity Professional v10.0.5 (build 42677)

Any tips/pointers would be very much appreciated.