If you update your NIM server to a new TL level and you do alt_disk migrations from this NIM server on client servers on lower levels, you need to update the bos.alt_disk.boot_images fileset in both the LPP sources and SPOTs you intend to use.
Say you have downloaded the TL level for your upgraded NIM server to /nim/lpp_source/lpp_source_AIX7204-02 and the older TL level you still want to be able to migrate clients to is 7.2 TL2, then you need to update the bos.alt_disk.boot_images fileset like this:
nim -o update -a packages=bos.alt_disk_install.boot_images -a source=/nim/lpp_source/lpp_source_AIX7204-02 lpp_source_AIX7202-02
List the updated fileset:
nim -o showres lpp_source_AIX7202-02 | grep bos.alt_disk
Now add to the SPOT from the LPP source:
nim -o cust -a filesets=bos.alt_disk_install.boot_images -a lpp_source=lpp_source_AIX7202-02 GOLD_AIX7202-02
And check again:
nim -o showres GOLD_AIX7202-02 | grep bos.alt_disk
Your SPOT should now be operational for alt_disk migrations again.