Not intending to slag DT1's, but I expect improvements were made to the DT2 and DT3 models?  I'm particularly interested in engine and transmission design changes that improved rideability and durability.

That would depend who you spoke to on a particular day.

a

I owned a 68,70 and 71 DT1 250 now and have owned two 73's, one 74 and three 75's. All the bike have changes. None that i aware of that actually improved durability. In 72 the transmission had shifting changes and torque induction reed valve added. Reed valve engines have improved throttle response and better lower in. In 73 Yamaha added a 21 inch front wheel for better handling especially in sand or loose dirt conditions. 

Some say 73 is the best DT250. I myself don't think so. You will get a different answer from everyone you ask. Between the 3 DT1's i have the 71 is the clear winner as for a riding bike. It's a heavier plusher feeling bike, has the later forks and shocks, different swingarm. I never did like the 73 Dt250's i owned. I do like the 75 DT250's and prefered that new frame design and how it handled on the street and trails. I personally like the DT1's for looks and nostalgia. The 75 DT250 for on/off road riding. I used one of these all summer around town and trail riding. Very good bike.  

Durability comes down to maintance and how you ride the bike. I don't think any have a avantage there. 

Enduros are like women. We all like them but different ones.
I solve the problem of choosing by having several. (Enduros, not wives, she won't allow that).
My favorite riders in order of best performance and feel:
'73 DT3
'71 DT1E
'73 RT3
'71 RT1B

In order of getting the most attention:
'68 DT1 (garage queen, doesn't get ridden)
'71 RT1B
'73 RT3
'71 DT1E
'73 DT3
Also the 250s are easier to start.

What "shifting changes" were made in 1972?

Pretty much everything inside the inner cases. Shift cam, all 3 shift forks. Different design for sure. They got rid of the big shift fork 1 that slide over the cam. They put the shift fork 1 on its own shift fork guide, same as the RT1 360 engines and DT250 for 72-73. They also eliminated the left/right shift shaft from 68-71 engines. Take a look at parts schematics on partzilla to see the difference. Bring up 71 and 72 DT250 and compare. Is it better? It's possible! Never had issues with either one though. 

I'd forgotten about those changes to the drum & forks & might be when they went to the fine tooth gears & a higher ratio top gear. Loved my first DT1 & had close spacing all the way compared to overdrive top on the fine tooth change--felt that way anyway. I broke the early box third gear big wheel twice [brute me clutchless racing]. But funny bit in later years & a box full of assorted [best of the rest parts], i picked a nice looking drum--all nicely machined on the surface & not semi rough finish & with good selector pin tracks. Luckily i bench tested it during assembly & had 1-2-3-5-4 due to that fourth fifth selector wrapping around the drum so pin slides half a turn out haha.
No12.
www.partzilla.com/catalog/yamaha/motorcycle/1968/dt1/shifter-1

 

There were changes to 2nd and 5th gear clusters according to Yamahas work shop tech bulletins too.
I know that because i have just read it on CMSNLs parts breakdown page for the 69 DT1.
I think if you want night and day difference, then the DT 250 MX ! One big advantage on that one is the breaks work! 
Loads more bottom end driveability and better gearbox and clutch actuation assembly, not to mention the single shock.
However that's not what you asked .
I have a 1 and a 3 .
To sit on , the DT 1 feels more purposeful . Ride is subjective i suppose.