it just restricts the depth of water you can comfortably operate the
dinghy in. Means you get wet to the thighs instead of the knees when you motor up to the beach.
In an extreme case the lowered centre of effort will also change the
dinghy's stability and planing characteristics (sometimes in hilarious but not usefull ways)...but with a 5hp motor that is not really going to be an issue for you.
If the old 5hp had a yacht prop on it, then consider going over to a prop more suited to pushing a
dinghy. It makes a huge difference.
Oh yeah, as a final possibility, it is possible to just buy a short shaft adaptor. about an hour and a half of tinkering means you can replace the internal drive shaft with the shorter unit and either unbolt or cut off (depending on make or age) the spacer on the exterior leg. Check the manufacturer's website to see if they make a coversion kit and how much it is.
Cheers
Sasha