I can tell you that as Canadian and British Ceremonial uniforms go, they're still being updated.
Take the Highlanders (Seaforths, Gordons and Camerons) for instance. They are an amalgamation of 2 units; the Queens Own Highlanders and the Gordon Highlanders. They merged in 2006. The Queens Own are an amalgamation of the Seaforth Highlanders and the Queen's Own Cameron Highlanders in 1961. And there are dozens more amalgamations dating back to the 1700s.
With each amalgamation, a new dress uniform was adopted, with elements of both of the previous uniform taken on, and typically with a new cap badge created from elements of the old units, or sometimes redesigned altogether.
Keeping with The Highlanders, their new cap badge is essentially taken from the old Seaforth Highlanders cap badge and the Gordon Highlanders cap badge with the Scottish Thistle added on.
As for their uniform, they wear the Gordon kilt, but the Seaforth (Mackenzie) trews (a style of tartan pants used for dinners and other indoor ceremonial events), and their pipers wear the Cameron tartan.
On parade you can see the New dress uniform with the new badge and tartan.
Yes, it's a completely new uniform, however it's designed to look like a ceremonial uniform that would not stick out in the Napoleonic era. No one is going to create a digital camouflage ceremonial dress, because that would defeat the purpose of ceremonial dress. It is, for all intents and purposes, a brand new uniform, but you would never know to look at it, and that's the intent.
Another good example is the Canadian Forces Reorganization Act (1968). Then-Prime Minister Pierre Elliot Trudeau passed a bill putting the entirety of the Canadian Forces into a single dress uniform. Gone were the air force blues, the navy blacks, and the army khakis, and they were all replaced with a single, forces-wide hunter green dress uniform.
This was widely loathed, and all of the elements strongly voiced their objections. Shortly after it was introduced, they elements wanted their own uniforms and ranks back, and they eventually got it, with a redesigned dress uniform that was unique to each element. The air force got back their blue uniform and wedge hat, but redesigned along the same line as the current uniform, and ditched the old belt, and this year got some of their pre-unification rank symbols and names back. The navy got back their double breasted blacks and only last year got back the executive curl on their officers' ranks. The army kept the hunter green uniform, but last year got back the pre-unification officer rank symbols.
So yeah, huge changes to dress uniform, happening within the last year.
Edit: I should mention that the impetus for the unification came from trying to make the Canadian Forces distinctive from the British Forces. Prior to unification, the Canadians used almost the exact same ranks, uniforms, medals, honours, etc., as the British Forces. In an attempt to make everything more uniquely Canadian, new ranks were created that added the maple leaf to rank insignia, and make everything feel more Canadian. The "Royal" was removed from "Royal Canadian Air Force" and "Royal Canadian Navy" to Canadianize it, and make it less British. Most of these moves were wildly unpopular at the time, and many have been subsequently undone in the intervening 60 years.
I'd ask a brief follow-up to what OP has asked: why are Napoleonic-era formal military dress regiments the norm? Why did stylistic updates end then in Canada and the UK (e.g. why aren't WWI era uniforms used as ceremonial costume?)
