I realise that this is a very old thread but I just sufferred this very same problem.. migrated an old DNN 5 site to a new server.  After the migration was complete, i just changed the web config for the db server name.  Thats all!.. and i didnt touch the machine keys/decryption keys.

But when i looked at the keys they were mysteriously different between the old & new servers.  In my case, i suspect that the problem was that when i launched the site after the migration I got a DB connection error & i was presented with a wizard to configure DNN as a new install.  I suspect that this wizard was responsible for changing the web.config.

The first time the site was launched on the new hardware i got a DB connection problem but DNN was not giving me a "real" error message just some generic connection message.  So i decided to go through the wizard part of the way through to the DB connection page.  When i got to this page I pressed the "Test DB connection" and got my "real" DB error (which in my case had to do with not setting up  SQL server to accept remote connections). 

So even though I did not go through to the end of the wizard i suspect that the wizard made changes to the machine keys in the web.config for me.

Anyway, thats my 2c worth.  I'm only posting here so that I have a reference to this problem in the future because this has happenned to me before but i couldnt figure out what caused the keys to change.

cheers