The situation is not as simple as you might think. Although the issue ultimately comes down to speed, the difference in speed can also cause a difference in position. One of the complexities in the situation you are describing is that it's not clear what new speed you are talking about. 24p is an imprecise term, although it can mean 24fps, it commonly means 23.976fps. That discrepancy alone can/will cause drift between the picture and audio, and that's before we consider the 25fps original.
The general rule is to shoot and edit at the same frame rate as your desired final render. If you don't follow this general rule then you're going to need to educate yourself about frame rates, their relationship to each other and how your software allows you to convert between them.
Probably the simplest solution for you at this stage is to complete your edit and audio mix at the original 25fps and then convert the whole completed 25fps render (video + audio mix) to 24p. Although this method may not give the best quality results, it is the method least likely to result in the audio not sync'ing with the picture.
G