Hi, first you need to verify if you have a MC75 or MC75A, and if the additional 4 units were MC75 or MC75A. They are different. If its a MC75, make sure you look for software that matches the part number, MC75x6 or MC75x8. Go to the support site to find out of you entitled or not by using the serial numbers.
There are GUI and functional differences between 6.1 and 6.5, do some web searches and you will have many hits on the differences.
I also suggest calling our support center at 800-653-5350 and have your lab until and one of your client unit full model and serial numbers to help navigate to the proper upgrade image if your device is entitled.
I cant seem to find any pages where I would put my serial number in, do you have a link handy?
I looked in the "about" page under settings. My developer unit says "Windows mobile 6.1 professional"
and my client units say "6.5 CE Classic"
My part number starts with MC7596