WCG can't care less about the host name... it's there for you for easy recognition. If the same device ID is presented (a number printed in the message log of 6 or 7 digits) and all other credentials check, the Result Status page will immediately print the new name. For instance, you had 10 tasks assigned to 'capitals' and you then named it 'CAPITALS', the prior assigned tasks will assume that external name immediately on the RS pages.

Whilst, if you did a clean install i.e. did not restore an old boinc-datadir backup of that specific *known* client before the install, chances are indeed that WCG sees a new device, also because in that case the host name was not the same. It is critical that the old datadir is in place and recognized by BOINC before the first connection is made with the servers, so the recommendation (written up in those infamous Start Here FAQ somewhere) is to do all that whilst the device is not connected to the intertubes and only when you're happy to see the old data in BOINC, to connect.

All that said, miracles can be worked by stopping BOINC, pull the internet cable, overwriting the new data dir with the old data and doing a BOINC reinstall (repair), check things and connect. Maybe the servers will say ''hello old friend'', wipe any old result info and pull new.

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