I've swapped or installed hundreds of preamps and it's a 10 minutes swap indeed, nothing easier to do, especially when new with a diagram or a schematic.
I've also (re)wired totally unknown preamps implying redrawing and/or understanding the circuit itself and also way more complicated electronics jobs, but it's been my job in the mid 90's early 2000's.
Sorry Mick, but if your tech can't do such a simple swap, he's not a tech at all to me.