I wouldn't be a happy bunny finding out 5 months down the line that your upgrade from L to XL hadn't gone through. I'm unsure what you mean about the problem with the account capped, but from VM's side of things isn't it just a case of resending a config file or command to the modem again and resetting it.
And as Darthlinux mentioned, ask for a refund for the difference between L and XL for the 5 months you haven't had the XL service. Plus if you're feeling a little cheeky a little compensation for the hassle of sorting the mess out

.