Reg- Device Replacement

Hi,

I am working on Device Replacement. I have a doubt in that i have 2 device categorys(dcat) having 2 devices. s1 and s2 , and another device category(dcat1) having a device s3. full installation is done for dcat. i tried replacing devices with in same device category s1 and s2 , but its showing ' Device not in specifications' . But s1 got replaced with s3 which is not installed. Is it that we cant replace devices of same device category?

Thanks,

Archana