Just be very clear. So, don't bother dropping hints, because that probably won't work. Make it very clear what you are asking, and the response you are expecting. For example 'I would very much like to X,Y or Z with you on Wednesday at 7pm. I would like you to confirm that you we will do that by Monday'.
I'm afraid that changing a plan is never going to make you popular, however you phrase it! Some people with Aspergers also aren't a fan of other people planning things, for various reasons. Unfortunately, the attachment to a routine and an aversion to following other's plans can mean that people with Aspergers don't involve themselves in a wide variety of activities. My father was like this, my Mum would have to organise all holidays and outings and even then he would rarely come along. I am a little like that too - my boyfriend has to suggest things and then I plan them, but we rarely go anywhere or do anything which suits us, but of course may not suit you.