Best practice to upgrade a Plone 3 theming product to Plone 4
If you want the traditional editing interface, old-style theme packages are helpful. If you have HTML resources ready from the designer, and want them quickly deployed, Diazo seems a fit. Anyway, Sunburst is the Plone 4 default theme package, you should check it out thoroughly and take advantage from the stock Plone packages whenever appropriate to minimize your efforts. 
Aside the theming, my experience on Plone 3 to 4 migration, it takes more time moving Archetypes to Dexterity, especially if your add-ons are based on Archetypes. Of course, your mileage may vary.
I have several sites in Plone 3.3.5 with its own theming product for each site, designed by an outside contractor long ago. Would it be best that I try to upgrade these theming products to Plone 4 compatible or should I use the new Diazo tool built into Plone 4 to create these different themes for each site?
Thanks in advance for your help.