They thought of it too, the swines!
Originally Posted by Bashiok
With the release of 4.2 all Valor Points will be down-converted to Justice Points, and all Conquest Points will be down-converted to Honor Points. To curb the ability to sell items back to the vendors after the conversion for the higher currency, we’re instituting a new sellback policy for items purchased directly before currency down-conversions.
Items purchased within the two hour window before the release of 4.2 will lose their ability to be sold back to the vendor when the realms are brought down for maintenance. No refunds will be granted for items purchased during this time, either by the in-game vendors, or our support departments.
As an example, I buy the Vicious Gladiator’s Battle Staff at 2:30 a.m. The servers come down for the release of 4.2 half an hour later. Normally I would still have an hour and a half after the servers return to sell the item back to the vendor. With this policy change though, upon logging in my item can no longer be sold back to the vendor.
This policy is only in effect for maintenances that include currency down-conversions, such as the release of 4.2. To ensure you don’t purchase the wrong item with the inability to sell it back, we recommend buying well before the final two hours so there’s time to change any incorrect purchases.
For more information on the down-conversion please refer to: http://us.battle.net/wow/en/blog/2880562
Meh, I was SO going to do this.