Technically a Z80 is a Z80 is a Z80...
The speed difference is what they are rated for. A, B, C, E, and H are speed ratings. The higher the letter, the higher the speed ratings.
You can use a Z80 in place of a Z80A and it might work. It might last years. It might, however, lock up intermittently or just outright fail from overheating.
You never know!