It's taken nearly a year, but the Umbraco 4 codebase had finally reached a point where I felt that migration of this site off of the, now dead, Umbraco 5 product line and back into a line that would continue to grow.
I stopped adding content to and maintaining the site about 6 months ago, as the Umbraco 5 platform on which it was built struggled more and more and the site slowed progressively as I added content. My attempt to add new elements to the site was halted when the development copy of the system started intermittently failing to spin up at all. At this point I knew that I needed to migrate but I needed to both have the time to invest in making the switch and I wanted to minimise my changeover time cost.
The New Year seemed like just the right time to address this; with the business taking on new services (more about this to come) the site needed to be reworked but I couldn't do any of this without switching over the platform first.