Hello Hugues
When using SLAAC, the addresses that are configured depend on the operating system being used. Specifically for Cisco IOS devices, the EUI64 process is used by default. This means that the actual IPv6 address that a Cisco device will obtain will depend directly on the MAC address of the device itself. If this device is then removed and another device is put in its place, because MAC addresses are unique, the new IPv6 address it will obtain will be different. If the same device is shutdown and restarted, then the same IPv6 address will be assigned to the device once again.
Well, it really depends on what you want. Typically, for network equipment, it is preferable to use a static address so that remote administration can take place over the same destination IP address.
I hope this has been helpful!
Laz