I migrated Online Casino Reviewer from a flat file html setup to wordpress. What I did was make a complete backup of OCR, then deleted everything, yep everything from the site. Installed WordPress, installed the maintenance plugin and activated it.
Installed theme and configured layout and css how I wanted it. Then started the laborious task of manually migrating the pages over one by one, being sure to 301 redirect the previous urls. Then deactivated the maintenance plugin.
A lot of work and I am still migrating some of the older pages over, but it will be well worth it.