It's possible
A fuel driven bus contains one motor, a lot of gearing and tanks for the fuel. All of these are irelevant for an electrical vehicle and would need to be ripped out. Luckily, most buses have their motor in the back.
An electrical bus has usually a few driven wheels in the back. This means the old engine bay could be used for the electrical motors. Mounting the batteries can be done on the roof, like it is done with gas fuel tanks, or under the walking floor. However, you also need to swap all other parts of the bus that rely on hydraulic and pneumatic to systems that work with the new motor. This includes doors and steering. While it certainly is possible to convert a bus, it is costly and complex.
It's easier to swap a bus between different fuels, e.g. natural gas or bio-diesel.
It's uneconomic
A typical bus has an average lifetime of about one decade before it needs severe overhauls. Overhauling a bus mid-cycle for the price of almost a full bus means you don't extend the lifetime of that bus significantly, but you rise its lifetime costs.
It's much more economical to sell off the used buses and replace them with new electric ones.
However, it's the simplest to replace the buses in batches as their replacement is planned anyway and swap over the buses over time. A partial and timed changeover also allows training maintenance and driving crews on the new vehicles without stopping the daily operation of the old machines. Also it allows to keep up the old operation in case the new buses have starting issues you did not plan for. An often overlooked part is charging times: refueling a fuel bus is a matter of minutes at specified places and can be planned for. Recharging an electric bus is a matter of hours, which can require re-scheduling and replanning all routes based on the reach you get from a charge. A gradual integration also grants more time to install the required new infrastructure, such as charging stations with roofing, without a requirement to be online for 300+ busses tomorrow.
