Converting D5 to D6

I've been developing a site in D5 for quite some time now.... result is a highly modified Marinelli 5.x theme. And now I'm looking at possibly going to D6 instead.
So... how hard would it be to convert my customized Marinelli 5.x to a 6.x version? Any advice? Maybe I'm best off, just switching to the 6.x version then doing my alterations over again?

Or might there be a relatively easy set of search-and-replace steps that would do the job?