Many mechanics can access and replace the oil pan with the engine in place. There's usually enough room to lower the pan from underneath.
However, factors that *might* necessitate engine lift are:
* Severe rust or corrosion: If the oil pan is severely rusted or bolted on incredibly tightly, getting it loose might be extremely difficult, potentially requiring more space and leverage than is available with the engine in place.
* Damaged subframe: If the subframe is damaged or interfering with access, it could make removal much harder.
* Mechanic preference/experience: Some mechanics might prefer to lift the engine simply for easier access and a more controlled environment.
In short: It's *possible* without lifting the engine, but it's not guaranteed. Assessing the condition of your car and your own mechanical skills will help determine the best approach. If you're not comfortable working under a car, or if the pan is particularly stuck, it might be advisable to get professional help.
