Converting '68 120 mph Rallye Speedo to 150 mph

The speedometers needle is carried by a magnetic gyro. If not for the odometer being inside it wouldn't need or contain a single gear. Changing the speed displayed has no effect on miles clocked.
From my experience with the trip odometers I have to think they are deleting you trip clock because either they don't want to deal with it or they are actually exchanging your instrument for one they have on hand that doesn't have that feature.
Accuracy in a speedometer is great for avoiding speeding tickets. If it shows anywhere near 70 mph , don't look at it again. Watch where you're going.
I can put a 150 mph decal on a 120 mph speedo head, then adjust its calibration just a tad, and its fine up to about 70 mph. The faster you go, the more inaccurate a cable to magnet driven speedometer reports. The inaccuracy is increased a little by the shifted scale on the alternate screen. To repeat, none of this is relevant to odometer and miles recorded. That should be considered a separate instrument for all practical purposes.
Electronic accuracy is found only in those instruments.