I can help with some of this, as I'm porting a C++Builder application that uses TNT from 2007 to 2009. The switch to Unicode in D2009 is overdue and welcome. However, it's unfortunate that the transition is probably easier for those who have NOT needed unicode in the past, and probably still don't. If, like me, you needed Unicode and used Troy Wolbrink's great TNT control to provide it, you have a rather more complex job...
The good news is that there's a new version of TNTControls from TMS Software which supports D2009. I haven't looked at this, but expect it is just a 'facade' layer over the native VCL components to ease portability. I'd consider that if your other libraries can be rebuilt to use it.
However, you may be better going back to native VCL controls, and the reason is string types. TNT control have always used WideString to pass Unicode strings back and forth, and you may well have WideString use scattered through your own code. This will work, but it's not ideal as WideString should really just be used for COM interop as it 'wraps' the COM BSTR type. Native unicode strings in D2009 are reference-counted and should be significantly faster.
If you do decide to replace TNT components with native VCL ones, you can use GExperts "Replace Components" command - or, maybe easier, do a search and replace in your .DFM and .PAS files (which you DO have in text form, don't you) to replace TTNT with T.