I'm really hoping that someone here can point in the right direction because nothing I've tried is working and my searches aren't turning up anything useful.

Here's what I am trying to accomplish and what happens...
I have a number of 85xx series OS5 themes which I would like to convert to OS6, as well as I have some 96xx OS6 themes that I would like to convert to OS5.
For the 85xx series OS5 that I want to convert to OS6, I have opened the themes in TB6 which works fine. But it will only let me export as 4.6.1 or 5.0. I have also tried creating a new theme with the same layout using TB6 and importing the theme, but it freezes TB6 up and I cannot do anything but close the program out.
For the 96xx series OS6 that I want to convert to OS5, TB6 will only let me export as 6.0. But I know that some devices, for example the 9650, also run OS5. But the themes were made using TB6 and I cannot open them with TB5.
Could someone please tell me how I can accomplish this without having to start the themes over from scratch as I have over 20 all together? Is there a guide to doing this that I am just not finding?
Any advice would be greatly appreciated as I'm getting nowhere on this on my own
Thanks in advance to anyone who replies!