727 slipping

Yeah deep pan came with new filter and extension peice. Just seemed odd that it seemed to shift fine the first few times and all of a sudden started slipping when shifting. Generally I've had people bring slipping transmissions over that have gotten progressively worse vs all of a sudden. Does the kickdown lever adjust pressure even when shifting manually? Could that not being adjusted right cause lower pressure and lead to slipping?

Probably the remaining old fluid in all the cracks and crevasses that still contained friction material. Once you drove it around the friction material was diluted = slippage. That is why you hear story after story of transmissions failing after a fluid change. Out the drain plug comes fluid and friction material in a worn transmission.