Susan, I'm not well up on this and I guess it would need Max to say if he could host a particular site.Aside from the space issues which Max has said aren't a problem at Mudcat, there could be bandwith issues is a site got a lot of traffic and other resource issuses - as an example, in my contract "monopoplising processing power" on the shared servers comes into the high usage clause.
I can't see any real problem with straight HTML pages, maybe a few links would need changing to point to the new site but off the top of my head, that would be about it.
If one the other hand we are talking sites where some server side processing takes place, you need a server running the supporting software. As an example, my forum is written in ASP (which is a mixture of HTML and a scripting language such as VBScript), so I need a site running a server that supports that - Microsofts IIS does, I believe WebsitePro does and I've heard of one Linux port but not every host would support it. It also uses an Acess Database so I need something that provides me with the capability to connect to that via ODBC (although other SQL compliant databases would work with little or no mods).
The bottom line to me would be - if Max supported those things, all that should be needed is a new database connection set up and the site would be running. Without ASP support, a complete rewrite would be necessary...
I'm sure there are lots of other considerations but that is my view based on my limited understanding of these things at the moment.
Jon