Well for those who updated today and used to share their Vita with family or perhaps a friend you are going to be met with the following message when you try to switch accounts:
The memory card inserted in your system is linked with the following Sony Entertainment Network account.
To use the memory card with your PS Vita system you must format the memory card.
If you format the memory card, all data on the memory card will be deleted.
So, for those who try to switch accounts will be asked to format the memory card, which will erase all data stored.
Sony never specified multiple accounts could be stored onto one card, but this may come as an inconvenience to users as they will have to purchase more memory cards. I guess Sony doesn’t believe in “sharing is caring” and just want you to buy more memory cards.
Remember to backup everything before doing the update (if you haven’t done it already) and pick up the classic titles.