but less often the other way around.
Strictly correct. It is best to read "less often" with its exact meaning rather than to assume much less often, or rarely.
It is only an issue where the change in specification is actually important to the operation of the circuit. Given the way people design circuits, it's more likely that the designer was familiar with part XYZ and just used the newer XYZA than he went looking and found that XYZ would not work and XYZA would (but that is what you seek to determine).
What it does do is get you focussed on the differences and you can determine if they are significant in this application.
For more sophisticated devices (like hybrid modules) the change in revision may require fewer (or more) bypass capacitors, or bias resistor changes, or extra functions on previously unused pins, etc. so looking at the datasheets for both parts and playing "spot the difference" is a good idea if you can do it.
Then there is the pragmatic approach. If you don't have the datasheets, the part is inexpensive, and the consequences of it failing are not highly significant, just replace it and see what happens (and that applies either way XYZ with XYZA or XYZA with XYZ).