Few issues that come to my mind.
1. Age difference between the two PSUs
2. Wattage differences (check to see if the one you want to put in has the same or more wattage)
3. Size (Normally, the power supplies are standard ATX units, but Dell and some other companies make custom ones, so they could be any size/shape.)
4. Connectors (Again, the PSUs are sometimes custom, so the PSU may only have enough cables for a certain type of computer)