In order to compute the angle change, you need to know the front-centre. I'm confused as to what are your frame dims, but it should be around 600 mm. Dropping the BB 8 mm would then slacken the angles by 0.8 deg. It's significant but not excessive IMO.
BTW, I'm not sure how you can significantly increase clearance. Lengthening the stays will help, but there is a limit to which you can indent the stays (they likely already have a round-oval-round profile?)
Tuz points out an approximate 0.8 degree slacking of HT and ST angles, not a lot.
With 55mm fork rake and 73 deg HT, trail is at 46mm, a bit short (for my taste) but nice for light front loads. The same rake at a 72.2 degree HT gives 51 mm of trail, still in a very good general range.
To recover the original 46 mm trail, increase the fork rake to 60 mm.