England returned to winning ways against the West Indies after a debacle in India earlier this year.
The Three Lions suffered a 4-1 defeat during the five-Test tour of India earlier this year. In McCullum's words, the debacle on Indian soil prompted England to "refine" their aggressive strategy and overhaul the red-ball side.
As a consequence, legendary pacer James Anderson announced his retirement from the longest format while veteran wicketkeeper Jonny Bairstow and Ben Foakes were overlooked at the expense of Test debutant Jamie Smith.
During the West Indies series, fast bowler Gus Atkinson also made his Test debut for England and young Shoaib Bashir replaced Jack Leach as the team's primary spin option.
The young trio of Smith, Atkinson, and Bashir played vital roles as England completed a clean sweep against the West Indies on Sunday (July 28).
"No changes are ever easy, are they?," said McCullum, as quoted by AFP. "But we knew we needed to make a couple of improvements as a side and we brought in some new faces."
He added: "You don't ask for instant gratification when it comes to them but the way the series played out you'd say the guys have certainly shown they're good enough at this level.
"What we've seen from Gus Atkinson, from Shoaib Bashir and Jamie Smith, is that international cricket is where they belong and they've performed accordingly.
"Sometimes when you lose you have a period of reflection and what we've seen is a team which has bounced back with a bit of a harder feel to it. I think we're a stronger team now than we may have been in the past."
Anderson called time on his illustrious career after the series opener at Lord's with 704 Test wickets to his name, the most by any fast bowler in the traditional format. Only spinners Shane Warne (708 wickets) and Muttiah Muralitharan (800 wickets) are ahead of him in the all-time highest Test wicket takers list.
However, England have retained Anderson's services as a fast-bowling mentor, with a grateful McCullum saying: "What Jimmy's given in the past couple of matches is quite incredible. He's fitted seamlessly into our group of coaches.
"It's never easy but he seems really content right now, it's great to have him and I feel very lucky that he's part of our group."
England would be keen to travel to Australia with a group of genuine fast bowlers for the 2025/26 Ashes series.
"With the bowlers now we've got some pretty good ammo (ammunition) that we are going to need on flat wickets when times arise," said McCullum.
"Gus has touched 90mph (145kmh), been able to jag the ball off a good length and also been able to operate a bumper plan for a period of time.
"The way Chris Woakes bowled was outstanding. Then you add the speed of Mark Wood, which is quite frightening really.
"Shoaib Bashir and Stokesy (England captain Ben Stokes) are in there too... We're not the finished article but this series is a step forward."
(With AFP Inputs)