Hi,
Im a bit late to this, the 'symmetrical' add-on is a none contracted add-on, it can be added to any contract for an additional £6/mth you do not need to re-contract. As its a none contracted add-on it can also be removed at anytime without changing your existing contract, as it runs month to month.
That was the good news!
The bad news is the call centres that are 'offshore' don't know how to apply it, unless you hit lucky and get through to a call centre that actually has access to the full range of Virgin add-ons. The one you are after is called 'Upload speed add-on', they don't seem to understand symmetrical speed! You also cannot have this add on if you are on any contracts that come under the 'Volt' branded services. I posted my experiences of this a while back under a title of 'Beware of being upsold a Volt contract' or something along these lines. It's in the Volt small print.
I went through all of this a few months back, it took a fair bit of perseverance, and eventually spoke to a Virgin member of the churn team based in Sheffield to get it resolved.
The upgrade your package does not seem to work with XGS-PON customers and the call centres will tell you it's not possible. This is due to lack of training and also their online systems don't present this available add-on for some reason, most likely it's due to the service being supplied through a third party 'Nexfibre', who are rolling out the XGS-PON network for VM.