Just been through this - wow, what an effort.
1) Book your complimentary flight. Be aware that Amex now holds the credit shell when you will cancel, or so they say, and will not tell you the VA credit number no matter how you ask. Also be aware that the original PNR will stay in place when you cancel (next step) and route changes are not allowed.
2) Cancel the flight you just booked. In my scenario I actually cancelled a year ago and nearly lost my credit note (12 months max!). I imagine you can cancel any time _after_ ticketing. Note that ticketing is sometimes not completed until after 6pm EST, if you cancel before ticketing is complete then its like you never booked, so, wait until you get your PNR/e-ticket confirmation email. ** The cancel/rebook fee is circa AUD$107 ... take note of this as it may make the whole process not worthwhile.
3) Look at Virgin bucket availability. What most of us probably want to do is rebook this flight as a flexi and upgrade it to Business - so check the Virgin website for your route and click the 'view in points' button to get a view as to whether or not there are any J class redemption seats available - note, these are "Business Reward" seats, not 'any seat rewards'. I think VA books J upgrades into "Z" class, though its nearly impossible to see how many Z class buckets are available (not even Expert Flyer can tell you). There does appear to be a direct correlation between Z and Business Rewards however - if in any doubt, ring up Velocity and they will tell you Z class availability over the phone with the disclaimer that "this is true as of right now and could change at any moment" (fair enough).
4) Ring Amex again and quote the original PNR, which is now your credit shell, or as Amex seems to call it a 'flight hold' (???) Tell them you want to use this credit together with new money to book into a flexi fare. They won't tell you upfront what the value of the credit shell is. If this is important to your planning then make a dummy booking - they will understand and comply. Don't complete your booking in this case, just follow their prompts and gain the 'upgrade to flex' value, then call back if you have additional planning to do (this is how I did it).
5) Ok, happy now that both flex economy and Business Rewards are available for the dates/route I want to travel with a decent idea of the upgrade cost to Flexi, ring Amex back and complete the booking. Ensure they put your velocity number into the booking. Be prepared! This isn't anywhere near 'free' or even cheap. The original complimentary flight is a saver flight at some sort of contract rate, after Amex takes the AU$107 cancel/rebook fee you will have relatively little left in your credit shell.
6) Wait for your confirmation email. This is a bit of a shocker actually because it can takes hours, or might not even be completed until 1-2-3 hours after 6pm EST. All the while you live in fear that J Rewards seats will sell out.
7) Email comes and gives you your PNR and e-ticket number. Go to the VA website and attempt your Flexi to J upgrade. If all is well, this should be completely straightforward. You should be able to see you ticket by either using your PNR or by logging in to your Velocity account. I had some problems however and it might be worth a note: In my case, the PNR was correct and related to a Flexi ticket, however, somehow the e-ticket was booked as a 'saver' still, and because they didn't match the website wouldn't upgrade me (and be careful, because if they _do_ match, but are both Saver then your points upgrade cost will be huge!). I had to ring the VA GCC to find this out. Then ring Amex back and have them correct the error ... all the while aware that the J Rewards were selling out ...
8) Ultimately, your Velocity account should now show the J ticket and probably the points cost, and, interestingly, the original cost in dollars!
I've got another one of these to do before end of November and will report back here - its about the same process as its always been but you have to know what to ask for, be firm (but not impolite) and have about 1-2 hours available.
On retail pricing I am about $300 better off then if I'd simply bought the Flex and upgraded to J with points without all this stuffing around (todays Flexi return PER-MEL = $1178 + CC fee). Without a doubt the 'saved' dollar value will vary enormously with original Saver ticket price. I'm expecting SC earn and FF points as per a flexi ticket as this has been the case in previous years.