install ros packages from synaptic without upgrade?

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I don't think there is good way to automatically upgrade a single package while retaining all the other ones, as the entire dependency tree of that package generally will need to be updated as well. You can try and build the package in question from source, but at some point or other you will need to update the dependencies as well.

Additionally, there is never a good reason to modify the default packages, because as you have experienced your changes will get overwritten. I am not sure if there is a guide or tutorial that explains this better, but in general you should only need to make changes locally in your workspace.

At this time, my best recommendation is to do full updates, but infrequently and as needed. Also avoid making changes to packages downloaded through debs.