Going from a Ches 14 to 19" wide boat is OK if the paddler is totally happy.

The shortest version of Frej listed on my plans is 481cm LOA (15.8 ft) and 54cm beam (21.5")

With that set of dimensions as a starting point, I get the following 'scaled' numbers:

So a 15' boat could be 20.4 inches wide, not 19". It makes a difference which length is used as the starting point for any 'scaling'.

That said, it would be useful to read Bjorn Thomasson's page on 'Alterations' which discusses the possible pitfalls of 'scaling' a design.
http://www.thomassondesign.com/en/building/alterations

Addressing Jay's point about stability (a very important parameter IMO), using a 90% scaling factor 'on everything' will result in a boat with only 65% of the stability but 73% of the resistance. So the price paid in stability may dominate the advantage in lowered paddling resistance.

Oscar Chalupsky is famously quoted as saying: "Nobody is fast when they are upside down!"