Dunno if it's been mentioned, but it is not financially viable to upgrade if you're on NSO already.
The pro rata discount they offer is 4p a day. However, 17.99/365 is 4.9p so should be rounded to 5p, as an existing member "perk"
This may not sound like much but thats an extra 25% discount. If you have exactly 365 days of NSO remaining, the discount is not 17.99 as it should be, it's 14.60. That's quite a significant difference between the price NSO is meant to cost and the discount offered.
I'm not going to get into the performance issues - they will likely improve - this is purely based on financial, and it's a much better offer for new users than existing.
Hope this helps those on the fence, or those with stacked memberships