I recall from reading an interview with Rod Smallwood that the cover for the NOTB was changed for the 98 reissue as the original printing from 82 was actually a wrong presentation of the background colour as featured in the original artwork by Riggs.
In the same interview, I recall reading that the band felt that the original 90 cover for NPFTD was a bit too comical and so the band took the opportunity to remove the grave-digger character for the 98 reissue.
Not sure about the reason for the changes for the debut album, but it may be similar to the reason for TNOTB.
I personally think the 98 versions are an improvement.