I did the same on a transition vanquish, no issue.
My mojo HD3 was designed around a 46mm offset, I originally built it with 42. Just switched to a 37mm Lyrik. Unless testing back to back, very unlikely to notice. Even if you did, within 29 minutes, you’d have adapted anyway.
It doesn’t make a fraction of the difference marketing people try to tell you it does.
I predict, in the next few years, long offset forks will disappear and the only option will be the short ones. There’s just no benefit to the manufacturer to offer so many options.
It’s only one part of what affects the steering of the bike, he could add a long stem to counter, move the sweep more forward, put more spacers on the steerer, etc, etc, etc. They’ll be fine with it more than likely, if not, do one of the above to get the right feel back.
My new HT has a recommendation for a 51mm offset. I had some 42mm offset Lyriks. Spoke to the bloke who designed it and he basically said ‘well we kept getting asked so we had to put something on the web site’ 😉 (I’m paraphrasing a bit here).
If I put a 51mm offset fork on it, I’m 99.9% sure I wouldn’t notice any difference.