I am about to undertake moving a Miva 4.13 installation from another hosting company to WH and upgrading to 5.5 at the same time.
I found the thread in this forum
That addresses the upgrade as well as very similar instructional pages on the mivamerchant.com site
I have downloaded the the module found on this page
that makes the provide.xml file necessary for the upgrade.
So far so good.
But what I don't know is where to copy the Miva 4 files on WH's server. There are two Merchant 2 folders on the old host's server. They must go into specific places on WH's servers for the upgrade to work, but I have not found it addressed anywhere.