I am going to hazard a guess its a problem with the billing system rather then the config server sending out to modem. My reasons are.
Online upgrades are always instant, so presumably doing changes via different method, ironically everytime I have upgraded online I carry on getting billed for old tier for many months after
.
Once I was with a guy on the phone helping diagnose speed issues (not through normal channels) he was able to change my config at a whim no 1 tier change a month crap, no 48 hr wait, instant everytime and the modem rebooted itself automatically.
Another occasion when I got downgraded by a mardy CS guy, cableforum intervened and a kind lady got in touch with me and dispite the ntl excuse coming from techs we cant upgrade you sir until next month, she manually changed my config, I believe she put me on unreg file and then to 10meg, of course both changes were instant.
Believe what you want to believe.