I'm not so sure you will have geometry problems.
By the time "dd" gets to see the disk the heads, cylinders, sectors have all been translated into blocks (numbered 0 to 19847598457987546 :-)
So when you drop in a new disk and transfer the image file over by doing the dd in reverse it should be fine (so long as the new disk is bigger .. not hard these days)
The only thing I can think of is if whatever the Amiga uses for a bootblock also stores info about the drive the bootblock is on ... so if you dd'd to a bigger drive, is there anything in the boot block that would complain ?
Also would the Amiga be able to see the left over space of the bigger drive as free ? Or would it just not believe it was there ?
Regards
Mark
