There is a moment in life that is perhaps even more difficult to accept as a reality than parenthood. It is, after all, the next step up. Grandparenthood is a tough transition to digest and adapt to. Yes, it is extremely exciting - but, it makes the Grandparent-to-be realise that they are growing old, and will now have to pass on the crown to the next in line.
I see this difficulty by observing my own parents. Although they are both excited [and show the excitement in different ways], I can see how much they struggle with the fact that they no longer are Omnipotent - and that I get to have the final word, not them. I would guess it is more or less the same as admitting defeat, in a way.
To some, becoming a Grandparent might be the one opportunity they have to redeem themselves and try to rectify any mistakes they may have made with their own children, To others, it is the perfect chance of taking all that was fun about parenthood and relive it all over again - this time without having to worry about being the "Bad Cop".
As difficult as it might be for you to wrap your head around your own impending parenthood, try and be as understanding as you can with the Grandparents-to-be. It might take you an extra effort not to bite their head off when they insist about doing things their way - but with a bit of patience, you'll manage to agree to disagree at first, and then eventually, find a solution.
I had to face this with my own mother when it comes to Childcare. She seemed to take offence at my mentioning Childcare Centres to her at first. She wanted to be the one to babysit for me whilst I'm at work. I tried to word things as simply and carefully as I could, on separate occasions. I showed her that I still had every intention of checking out some Centres - and went into detail about one of them to make her realise why I consider it to be a good option to have. By the end of it all, she ended up telling me she now realises that yes, I should go for it.
My siblings and I grew up with our Grandparents and Great Aunt as our babysitters. I suppose mentioning Childcare Centres to my mum felt as if I was trying to rob her from "her turn" at living through the experience.
I don't really have any advice to share - other than to be patient. You have the final word - but it doesn't hurt to ask for the Grandparents' opinion in certain matters - it will make them feel useful and included.