FormFlow 99 to LC Forms 6 was basically changing to a new product. LC Forms 6 to LC Forms 7 is just a version change. The API's are backwards compatible, so you shouldn't need to make much changes code wise. One thing I can think of is LC Forms 6 renders PDF 1.5, and by default LC Forms 7 will render PDF 1.6. So if you require PDF 1.5 (ie: your users are still on Acrobat 6.x) then you'll need to add the otpion "PDFVersion=1.5" to your renderForm call.
Of course, if you don't make changes to take advantage of some new features, like the renderAtClient option, you won't see as good performance improvements as someone who has designed their application with LC Forms 7 in mind.
Chris
Adobe Enterprise Developer Support