Adrian
I am going to jump in and answer this. Typically simple key changes are 1 semi tone up, and particularly if there is a vocalist this makes sense because to move more than a semi tone might be stretching the vocal range to an uncomfortable point. In an instrumental, of course that is not an issue, but the simple 1 semi tone up also works quite well to give the song a lift. And it is easy to do, another consideration if you are going to use the transpose button, because its a single touch and keep playing in the same key but now the song has moved up a semi tone.
But when playing instrumental you are not restrained by vocal range, so in a sense anything is possible. And 'how' is more important than 'how much'. Players will use transitional chord sequences to lead into a change, -- say you are playing in C, you slip in a chord sequence of C: A7 and now you are into D - a full tone step. Of course that is a bit difficult using the transpose button, so is more likely to be done by an advanced player who will now play in D, not use transpose and play in C.
When nicely done using 2:5:1 type chord sequences, key changes of unusual intervals can also be effective. In a sense there are no rules, its about what sounds good. One song by the 4 Seasons for example changes key about 5 times. But typically- like most other things - it is best not to overuse key changes.
Mike