Yes, it can be done. This is if the 7.0.27 was installed from repositories. Just download the 7.0.42 version and unpack it in your home directory. Now you need to edit the file in /etc/default/boinc-client to include the location of the new version. Also, this should only be done on the same major version, i.e. v6 to v6 or v7 to v7. Due to the differences in v6 & v7 client_state files, you can't use it to switch between v6 & v7.

The following steps should work.

Open a terminal and type:
sudo service boinc-client stop

sudo gedit etc/default/boinc-client [Note: gedit may not be the correct text editor on 12.10, so use the appropriate editor.]

Look for the section that starts with # This is the location of the BOINC core client, that the init script uses.
At the bottom of that section, you will see an entry BOINC_CLIENT="usr/bin/boinc"

Create another entry below that like #BOINC_CLIENT="home/username/BOINC/boinc" of course replacing username with the actual directory name. [The # will tell the init script to not use this entry. So whichever version you don't want to use, just put the # in front of that entry and save the file. Always make sure one of the entries has the # in front of it.]

Save the file and exit the text editor.

sudo service boinc-client start to start the boinc client again.

Ok, in short: I got the BOINC_7.0.42 installed with the BOINC_7.0.27 seem to be intact.

Long story:
I had some problems along the way. I ended up using a mix of the command line and file-explorer operations. Also, I can't get the two BOINC-managers to work at the same time; just thought I may have use for that, but thought later that it'll probably be too complicated.

The app_config.xml for 6-true-cores AMD-1090T:
4 SNS2 tasks
3 HCC tasks (1-HCC-for-CPU, plus 2-HCC-for-GPU)

<app_config>
<app><name>sn2s</name><max_concurrent>4</max_concurrent>
</app>

<app><name>hcc1</name><max_concurrent>3</max_concurrent>
<gpu_versions>
<gpu_usage>.5</gpu_usage>
<cpu_usage>.5</cpu_usage>
</gpu_versions>
</app>
</app_config>

I'm learning the logic involved and with that, makes the app_config straightforward to use. Now I'm set to explore and probe where the maximum efficiency lies in the number of simul HCC-WUs for the HD7770. My thanks to BobCat13. Case: Resolved.
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