I think the thread heading should read
Converting Internal 2.5" drive to 3.5"
You can't physically reduce the drive down to a 2.5"
You can however get a 2.5" to 3.5" conversion kit.
It comes with the brackets to install a 2.5" drive into a 3.5" bay as well as all the relevent cables that you will need.
Something like 2.5in to 3.5in adapter bracket, lets you mount 2.5in HDD/ SSD in 3.5in bay, comes as a pair
I think thats what he meant or he would have said converting internal drive (impossible) from 2.5 to 3.5.. but he said from 3.5 to 2.5 using a 2.5" ssd..
I have an SSD In Mine Very Very Happy!!! You can get adapter which screw onto the side of the 2.5" and then it brings the width to the same as a 3.5" drive Simple. All SSD's AFAIK are Sata, imagine and IDE SSD LOL. But good work going with the ssd, Which one did you get?