Upgrade PAS 3 or PAT 4?

After my father passed away I inherited his audio system and components. My question is quite simple really, which unit should I upgrade, the PAS 3 or PAT 4? Both units are in average to good condition and work but are very much as they were when he built them.
I would appreciate any recommendations and info as well as why either unit is preferable to upgrade. Due to my work I am comfortable with schematics and have good solder and component skills so doing the work is fine unless the belief that pre stuffed boards are a better choice.

From a sonic and performance point of view which unit makes sense to upgrade. It is not really about the value for me. I am looking at the sound and what it might bring to my system. I have a Sonagraphe SG3 turntable with Grace 707 arm and Micro Benz cartridge on one end and Spendor 2/3 speakers on the other end.

it depends on what you mean by "upgrade". If you want maximum performance at the best price, I'd say sell the PAS3 and gut out the PAT4.
You can drop in an SP8 preamp board and a transformer for a total of about $250 and have a fantastic sounding preamp that looks all original.
You could also do that to a PAS3, or you could sell the PAS3 for about $250.
I've had customers spend over $1000 to upgrade a PAS3, but it would have been less expensive to build a new preamp.
The SP8 circuit sounds alot better than a $1000 upgrade, it's a matter of carefully choosing parts versus value for $$ spent.