The transistor slots in the board for the 1446 transistors are not large enough to fit any 2879's in there.
Look at the data sheets for the two different transistors you are talking about and you can see the physical size difference.
Now then, can it be done ???
Absolutely.
Not even cost effective because the board will need to be removed to resize the slots, possibly relocate the transformers to accommodate the larger transistor slots.
A bunch of "stuff" would have to be changed.
Not even worth the time or cost.
As mentioned above, sell it and get what you want.
If I was doing the conversion, it would cost at the very least as much as a new amplifier.