If you are worried about breaking the clips, just order some. They are pretty cheap and it is hit or miss if you will break one. Pretty much gaurantee you that if a stereo shop breaks one, they won't replace it. You likely won't ever know unless you take the panel off again.
I've had my rocker panels, and several other panels off inside the car and keep a handfull of all the clips on hand just in case.
From Real OEM, I have listed the ones I think you might need for a M3 below; part numbers linked to ECS Website:
07147152593 - 14 on each front panel; 2 on each rear panel
07147441715 - 2 on each front panel
51418224768 - 20 on each front panel; 16 on each rear panel
For clips that there are only a few, I ordered 5; for the clips that there are lots of them, I ordered 10-20 of. I ordered some 12 different clips and have not run out of any of them. You could always go to the dealer to get one clip, but for $5-$10 dollars for each handful of clips, it's just not worth the hassle for me. All in, I think I spent $100 to cover all three of my BMW's.
Changing speakers is really easy, go for it.