The HRDlx III is one example of build/quality improvement over the original MIA version. The III has thicker circuit boards than the I or II, has a couple of other improvements, such as added audio taper pots and some minor circuit revisions, and is otherwise identical in construction and parts. The new RoHS crap solder being a possible exception.
Many MIA amps already have some parts MIM, and have for a long time. The cabs are a good example, many have been MIM for years. Typically, the exact same brand components are used in either build, meaning that some MIM amps have MIA parts, such as the transformers.
My BJr II, another example of improved over the original MIA, is MIM, and it's been great. The MIM version started with a circuit revision that improved the reverb, changing the point at which the reverb was inserted in the circuit, reducing noise and other anomalies. I have A/B'd the MIA (I) and MIM (II) versions side-by-side, I prefer the MIM sound much better.
The improvements on the III models was part of my decision to purchase one now. Being able to afford a decent tube amp was another factor. I do like the ability to be able to run a 4 or 8 ohms also. Some of the modern amps do not have this flexibility.