The upgrade file you are looking for is here:
"Pdaxii13v2, upgrading to build 5.5 alpha4"
https://www.oesf.org/forum/index.php?showtopic=24702pdaxiiv2-upgrade_5.5.0-alpha4_armv5tel.ipk
When Meanie comes out with an updated version, he updates the link in the first post, and modifies the title a bit.
Also in post #12 canguy247 mentions upgrading an akita that has the full version installed to a SD card.
On the last page of this thread, Meanie says there is no base pdaxii13v2 image.
I don't think Meanie intends to make pdaxii13v2 for the akita model.
I have pdaxii13v2 installed onto my C3100 completely on NAND.
I manually removed large packages and installed them to my HDD (I have 29MB left on NAND)
There is someone else trying this also on the last page of that thread.