There are 2 types of downshift. One is as you reduce speed, the transmission shifts down. The other is under WOT or acceleration to allow a faster rate of speed increase. There are very different operating conditions for both.

On the transmission shifting down through the range as you decrease speed, usually the throttle is mostly closed and you will have "high" vacuum to smooth out the shift. Under a WOT or acceleration downshift, there will be little to no vacuum to firm up the shift.

If no vacuum is present for speed reduction downshifts you will get clunks and jerks as the transmission shifts down (very abrupt shifts). If too much vacuum is present on a WOT or acceleration downshift, you will get a flared shift or a slipping transmission.

The vacuum situation is complicated in these cars. On a gas engine the vacuum is a direct proportion of load and throttle position. The diesels have to "fake it".