Can you post the entire itinerary? MSL may not allow for tickets in that class to be sold, even though it's showing available for that single segment.
There may also be some confusion with the term "upgrade". Since it sounds like you're willing to pay, I think you should ask to change the fare class.
MSL aside, there is plenty of room in business on 4464:
Code:
US (**) 4464 YYZ 7:15a → CLT 9:28a
J8 R8 D8 I8 C6 U0 Y9 B9 H9 K9 M9 L9 W9 V9 G9 S9 N9 Q9 O9 E0 T0