If you email support, they will work something out with you. They should have an upgrade path, but the way Vienna (in my opinion, mistakenly) looks at it is, going from VI to Synchron is an upgrade, going from Synchron to VI is not an upgrade. The one thing I can tell you is, regardless of their policy and what they say on the forums, they are interested in making a sale and they will give you a good price, probably near equivalent to the upgrade from VI to Synchron path.
As to whether it is worth it, in my opinion, it is. Synchron is great for having an out of the box good sound. Synchron however is very epic, Hollywood movie soundtrack sounding, which is not always what I am looking for. VI on the other hand provides a more subdued sound that I can, if I choose, make sound more Hollywood'ish, or, I can use less a less "epic" timbre for when I am writing for a more "classical" feel. After all, Im not always looking to have people feel like they are listening to a soundtrack.
The coronet is one of the special gems in the VI collection, by far, it is my favourite solo instrument, followed by the classic oboe, and the flexibility these instruments offer in the VI version far surpasses the synchron versions. By having a less wet and heavily eq'd version of these two solo instruments, I am able to use these two solo instruments for a more soft and mellow tone than what the Synchron versions allow.
I might also add, VI is incredible with MIR Pro.
The biggest issues with VI instruments however is, they dont sound good out of the box. They will require some pre-tweaking. If you are willing to spend some money, Vienna provides easily set presets that make them sound good, but it will cost you. For starters, Instruments Pro ($160) is a must with VI instruments. Given that they are giving away the features of VI Pro in Synchron, I am still unsure why they are selling VI Pro, but it is what it is. The premium features offered in VI Pro range from humanization to (unbelievably, you have to purchase VI Pro for this next feature) polyphonic legato, and some very useful functionality like time-stretching. You will also want either Ensemble 7 Pro or Vienna Suite for the useful eq and compression presets. I would recommend Ensemble 7 personally, as I dont feel like Vienna Suite offers anything appreciable that Ensemble 7 Pro doesnt offer, especially for the 3x price difference. Finally, you will need a good reverb engine. MIRx is ok, albeit quite expensive for what it is (almost zero settings & cannot be used outside of Vienna Instruments), but it sounds quite good. It is also worth noting that they still do not have a Synchron stage MIRx roompack available, so it is a bit tricky getting VI instruments to blend with Synchron instruments if you are using MIRx. MIR Pro on the other hand is the top reverb engine choice for orchestral instrumentation, but is the most expensive reverb engine on the market, with altiverb coming in at about 66% of the price difference, and, MIR Pro offers a Synchron roompack, so you can make VI instruments blend perfectly with Synchron instruments.
In short, be prepared to spend some serious cash on VI instruments, outside of the purchase for the VI instruments. I have both Synchron and VI instrument special editions 1-7 and 1+ and 2+, VI pro, VEP 6 Pro (three licenses), VEP 7 Pro (1 license), Smart Orchestra, Smart Spheres and its addon, MIRx complete room pack, and MIR Pro unlimited + all of its roompacks. I only mention what I have purchased licenses for so you know that everything I am saying is coming from experience. You can expect to spend an additional $320 minimum for Instruments Pro and Ensemble 7 Pro at a minimum, then potentially another close to $500 for MIRx complete, or $1600+ for MIR Pro complete, but what you get in the long run is far more flexibility than what Synchron offers, at the cost of price and time that must be spent tweaking each instrument to make them sound good (which is much easier / faster to do with the purchase of Ensemble Pro and MIR Pro).