LHC@home

Go to your LHC account and find the old host ID you want to use
Write down its ID number, then go into that hosts details and get the number of times it has contacted the server

Install BOINC on the new computer as per normal
Disconnect that computer from the Internet (unplug ethernet cable works)
Copy across the old LHC account file from the old host (or any other host with LHC) and place it in the BOINC folder of the new host
Start BOINC or restart BOINC (still with internet disconnected)
Then stop BOINC again.
Go to client_state.xml file and open for editing
Scroll to the project you want (LHC in this case)
Locate the line that says <rpc_seqno></rpc_seqno>
this is the number of times it has contacted the server, write the number you copied from the old host details between the >< and add 1,
so if the number is <rpc_seqno>2300</rpc_seqno> then you change it to 2301 on the new host (this important or it wont work)
Then locate the line <hostid></hostid>
And add in the id number of your old host.
Save the file.

Now you can reconnect the internet and restart BOINC and you should be reusing the same Host id you had before.

Might have to try this since I have one that is still the same as before but they won't let it merge.