EXCELLENT question. Answer is no, they will not be disconnected...so here, let me give you simple example, because I did this few times with customers and there were no issues. Say you have default OM subnet 172.16.10.0/24 and for argument sake, you decide to use 10.10.10.0/24 instead. If you change it and push policy, users will NOT be disconnected and they will keep having same OM mode (172.16.10.xx), but once they reconnect next time, they will get new OM mode subnet IP from 10.10.10.0/24 subnet. Just ensure new subnet is included in same places as existing one (config, rules, etc...).
And no, they will NOT have to delete/re-create the site either, which is always a good thing, as most users hate doing that (well, at least from my experience : - )