I had similar happen to me - banner popped up about 2.5-3 months before my contract end date. I contacted VM and they said the same thing, that I wasn't due one yet and that the system sometimes had glitches. Sure enough the banner went away shortly after.
Then it came up again 2 months before contract end, so I asked again and was then told I was allowed to upgrade to a new phone at that point, but had to wait until the final month to switch to SIM only (which was what I wanted to do).
In the end though I rang them 2 days before I entered the last month, and they switched me to SIM only in time for the next refresh date anyway!
Sounds like the an extent, they offer you what they feel like offering!