Excellent info. I worked for Dyna from '95-'98. At that time, the Mitsubishi Meldas controls (M3 & M520) were being phased out and replaced with Dyna's PC based control. With the Mits and PC controls, Dyna was aiming for the bigger machine market with the largest mill being 43" x 22" travel 40 taper 15hp 16,000lb VMC.
The SKIP control was designed by Sandy Walker at Dyna. It was originally intended for the small benchtop machines Dyna sold. Over time, the SKIP control got adapted to bigger more powerful machines like the 4000/4400. The PC3 control that Dyna eventally released was also originally designed by Sandy. Unfortunately, Sandy passed away while the PC3 was under development and before it was released. The PC3 control is, to a great extent, a SKIP control with servos instead of steppers and a PC for the user interface instead of the proprietary console used on the SKIP controls. Because it was PC based, it used conventional G & M codes, had a hard drive and floppy drive, and color LCD screen. Also unfortunately, Dyna started selling the PC3 machines too soon before all the bugs were sorted and alienated their customers and dealers.
For reference, see attached pictures of my Dyna 4400s with the Mitsubishi M3 controls. These are mechanically the same as the SKIP controlled machines but take advantage of the (at the time) more modern Meldas control with higher rapids (400ipm vs. 100ipm) higher spindle speed (10K rpm vs 7K rpm), rigid tapping, more memory (32KB), and standard G & M codes.
As far as I know, the ballscrews, spindle, ATC, and basic iron are identical between the Mitsubishi controlled machine and the SKIP controlled machine. This reinforces the statement that the early machines are good candidates for retrofits.