I also spoke with WD and they say that
1. The SATA II drive will be a straight swap
2. The SATA III drive will be a straight swap, but their is a caveat. The SATA III drive has jumpers which are normally factory set for desktops and workstations. However, if you have problems then set the jumpers on pins 5 & 6 which should cure the problem. Article 981 on the WD knowledgebase has all the details on how to do this.
WD recommend using the SATA III drives, not only are they are newest, but it does mean that if you want to reuse the drive elsewhere in future its more likely to fit.
Many thanks to all the replies about this question!