Okay, I'll post about it again...
1. George Lucas is INFAMOUS for changing the story to his whim ANY time a continuity issue comes up.
2. This has been posted here before, just as every other subject you have brought up so far... Are we getting the point yet? ;)
3. Put it in perspective...
All design changes aside, or from what Lucas says. The progression from the ROTS hilt to the ANH hilt is actually quite natural...
A) The Anakin ROTS was a saber built in the time when the Jedi were at their prime. Before Order 66 and before The Purge. Parts to build them were still pretty readily available.
B) Anakin's ROTS Saber Was subjected to quite a bit of action in this episode, from fighting to "Save" the Chancellor, VS Droids, and Grievous's Personal Guards, To a fight on a volcanic planet. By the end of EP III this saber had most likely seen quite a bit of damage by the time Obi-Wan salvaged it from the Ashen surface on Mustafar. (This was a customary Thing to do when defeating a fallen Jedi that had turned from the order. While the Jedi's weapon is of their own design, it is STILL property of the Jedi Order and must be reclaimed, typically by the Master that trained the other Jedi.)
C. Since Order 66 was carried out and all Jedi believed Dead other than Yoda, Obi-Wan, and the Offspring of Skywalker. There was no Jedi Order. Obi-Wan Kept it to salvage.
D. Once he realized after speaking with Qui-Gon Through the force that Luke must be trained, he began upkeep on the saber.
E. This is where most of the Cosmetic changes can be explained: Due to the harsh environment of Tatooine, Upkeep of a saber is nearly tripled. Remember what Anakin said to Padme in Episode 2. "I hate Sand... It's irritating, and it gets EVERYWHERE!"
F. By the time Obi-Wan had settled in and begun his vigil over the Lars Moisture farm, the saber had to be repaired.
Now let's go into what changed and give a possible explanation:
From Anakin to Luke
The saber had a covertec knob when Owned by Anakin. As we stated before sand has a notorious habit of getting into crevices, and causing wear and tear. In a desert climate a covertec would have been prone to a LOT of wear and tear... The D-ring has much less area for wear since sand can't catch in it as easily. i.e. Blows right through. D-RING EXPLAINED
The Anakin saber had a closed switch box and clamping band. This part seemed a bit flashy to me, and could easily break. Obi wan had to use what was available to him to keep the saber up to par. So he replaced the clamp collar and lever with available parts. This made the saber easier to field strip, should sand get into it and muck up the inner mechanics, and electronics. CLAMP EXPLAINED
The Anakin Saber's Copper/Gold buttons probably wore out. Leaving Obi Wan with no choice but to change them out. ANY switch will wear out over time. Even Blade width and length adjustors. Once again direct replacement parts probably were not readily obtained, so similar scavenged parts must have been used. RED BUTTONS EXPLAINED
Anakin's saber had six grips. These were most likely replaced after the showdown on Mustafar because they were Scorched or perhaps could have been damaged by the hostile climate of Tatooine. Rubber DOES get brittle after losing it's moisture Content. Perhaps as he got on in years Obi Forgot how many grips his old Apprentice had on his saber, and added an extra... Who Knows? 7 GRIPS ON ANH EXPLAINED
Anakin's saber had a stepped pommel end, for a more finished look. Perhaps the original rear casing was damaged and needed replaced at some point. Perhaps to allow better fitment of a D-Ring? Therefore the change in appearance. POMMEL END EXPLAINED
The Rabbit ears on Anakin's saber appear to not connect to the emitter matrix retention ring. Perhaps the emitter matrix suffered some damage and had to be retrofitted with hand made parts. Once again, a parts availability issue. BUNNY EARS EXPLAINED
Anakin's Saber had an electrum switchplate, Perhaps this was damaged or wore out over the 18 or so years of non use or Desert climate issues. Perhaps Obi-Wan needed a better way to seal the activator? BUBBLE STRIP EXPLAINED
Anakin's Blade was a DEEPER Blue. Perhaps the Crystal wore out, or was changed somehow during the battle on Mustafar. Or we can use the Desert Climate for an example here. Perhaps the contacts inside the saber had corroded, changing their effectiveness and performance? CYAN BLADE COLOR EXPLAINED.
And for the FINAL Explanation:
The Bottom line about the changes? COPYRIGHT PROTECTION! You see, Lucasfilm CANNOT copyright the GRAFLEX based hilt as his own, EVEN WITH the changes to make it a lightsaber. The rights to that flash are Still Owned by Graflex (They make Night vision e-scopes for Tanks now!).
If he makes significant changes to the design, and changes the outward appearance of the body itself. It becomes HIS design. Therefor he has legal recourse if someone copies it... It's as simple as that.
EVERY saber in the PT is covered by this Copyright, as intellectual property.
This is why the Obi Wan ANH is such a grey area. AND the reason for the changes to its design as well for ROTS.
PLUS a custom designed hilt was probably FAR more cost effective to make a few copies of and just put them through their paces. A Graflex on set would be a NIGHTMARE to deal with for duelling shoots. Plus that is extra Labor hours paid at UNION wages for a props master to take extra time every shoot to make sure the hilt is serviceable...
I suppose you have to look at this from the perspective of a SABERSMITh, to be able to envision the STORY explanation, by using a knowledge of electro-mechanical limitatio0ns, and need for service on a weapon.