Re: Mercury 6hp to mercury 15hp foot swap?
If you are simply swapping the gearcase (leaving your existing leg and water tube in place), then it is a straight swap.
The gears are the same (2.08:1), the driveshaft is the same, etc, etc.
The ONLY difference between the pre-95 and post 94 1/2 units was the water tube and the exhaust plate.
So, if you retain the driveshaft housing and water tube (with the powerhead) of the motor you want to swap with and simply bolt on the lower unit, it will be fine.
From 1986 to the end of production the 6, 8, 9.9 and 15 horse were identical when it came to the lower units - I have swapped them many times...
A "professional" is someone who gets paid for their work - it doesn't necessarily mean they are good at it :)