SEP is a weird alien it's hard to predict what it does are it is going to do cause most of the time the documentation is just the opposite of whats happens. Sorry to spoil the thread.
The documentation stated that it aint possible but i just moved client from 18 SEPM server and huge chunk of about 20000+ clients of which non on them where in version 12.1.5 always where below 12.1.4 majority in 12.1.2. I was able to use HI policy and move all of them to just one server.
I did face issue with one server and the issue can be seen in the follwoing thread
https://www-secure.symantec.com/connect/forums/sep-1215-does-not-update-nac-definitions
@Outrageous - For your question will SEP 12.1.5 Server be able to update HI policy to 12.1.4 or below client yes as per my experience and as per a tech in SEP support but if you have existing SNAC policy in place and if your gonna upgarde from 12.1.4 or older to 12.1.5, then its advisiable to contact support to find out what can be done.
~Edit~