See this forum post:
In regards to your custom tags I'd recommend a lot of testing. Versions 6 and higher of CF are written in Java rather than C++, as was the case for versions 5 and lower. I suspect that if you have any migration pain it will be related to your custom tags. However, I cannot speak from experience since I have not used C++ custom tags.
We have a number of old "legacy" C++ custom tags from our CF 5 days which seem to work fine running in CF 8. I believe Adobe has maintained backwards-compatability with previous CF tag formats.