Admittedly, we haven't seen your system and have no knowledge of other factors that may be playing into your decision to replace a 984 with a PLC/5, so we are not really in a position to second guess it, and that is not my intent here.
The plus I see for the PLC/5 is that it is still on the market and still supported. You can buy a new PLC/5 from the factory, whereas if I recall correctly, 984s were discontinued circa '98. However the installed base of 984s is so huge that spare parts, even never been used parts, are really easy to come by and will continue to be easy to obtain for many years. We've still got 12 racks of 984s with 800 series IO in operation and haven't had any issues obtaining parts. So the fact that the the PLC/5 is still being manufactured counts for something, but not much. There is something significant to be said for the tech support for the PLC/5.
A justification could be made based on modernization for a ControlLogix, but I don't see a justification for replacing a 984 with a PLC/5, even though the PLC/5 continues to be one of my favorite PLCs to work with. Depending on what you have, you could probably buy a new spare of everything in the 984 system for less than the price of just a new PLC/5 processor.
edit to add:
I'm pretty sure that Proworx NXT can import the Taylor software files. We converted up to NXT in '99 so I don't completely remember, but (insert memory disclaimer here) we were able to import without doing any export from the Taylor software. I can't say if Proworx32 will work or not, besides you want to stay away from Proworx32 if you can. PLC Workshop 32 for Modicon will not import from other packages - you have to upload the code from the PLC and you have to export the comments from the original before importing into PLC Workshop.
Just my 2¢.
If you do go ahead with the conversion we'll be more than happy to help you - I might be misreading you, but I get the sense from this thread and your other thread that you are more anxious about than you need to be. Its not difficult, there is nothing that the 984 can do that a PLC/5 can't also do, and the strong data typing of the PLC/5 makes many things easier.