It might have been a gamble once, when "managers" literally ran football clubs, but the whole point of having Wilcox, Ashworth, Berrada, Blanc, Brailsford etc...is that they're the one's running the club...and the manager is "just" a coach.
If McKenna is a better coach than EtH, and I'd say there's strong evidence to suggest he is, then make the swap.
If it doesn't work out...you sack McKenna and appoint the next up and coming young coach...rinse repeat until you land on the right one.
It doesn't need to be any more complicated than that.