Welcome to the forum Andy H!
It sounds as though you have been hearing rather negatively about the food you currently feed your boxer viz i have had lots of stick because this is in effect junk food
... and yet there are plenty of people who feed their dogs on foods which others may disagree with, even 'disapprove' of - some fed unknowingly, and some albeit deliberately - for reasons not always apparent to others. Amongst other reasons are limited cost considerations which reduces choice, or breeders who affiliate with a specific manufacturer, more commonly particular foods being donated to charities and sometimes working dog sponsoring by manufacturers; and in these cases food choice is limited to dogs.
Indeed in times past pet dogs were given food that was absolutely the norm, and nowadays would be thought of as perhaps 'junk' food. Trends change, and together with our greater understanding of what we feed (and what we learn from each other) we are able to become better informed when choosing what to feed our dogs.
Firstly I'd ask you to think about which of the Nature's Menu you'd want to feed him as some are complimentary
raw foods and some are complete
It may also be helpful to look at the nutrients
of the food you are currently feeding to your boxer, and Bakers Complete Adult has borderline average to low
protein, and borderline average to low
fat, borderline average to low
fibre, mid average
ash, and high above average
carbohydrate as shown in the allaboutdogfood (useful) nutrient dials here: https://www.allaboutdogfood.co.uk/dog-food-reviews/0012/bakers-complete-adult
Also to consider is the nutrients
profile for the food you want to change to. So if this is Nature's Menu Adult Nuggets (a raw complete food) the nutrients are mid average
protein, borderline above average to average
fat, mid average
fibre, borderline below average to average
ash and borderline average to above average
carbohydrate, as shown in the nutrient dials here: https://www.allaboutdogfood.co.uk/dog-food-reviews/0229/natures-menu-adult-nuggets
You can see, and as you might suspect, the nutrient profiles of the 2 foods are completely different, and together with different ingredients, plus moisture levels of approx 8% vs 70%, I'd advise a slow, gradual transition.
Hopefully, he will enjoy his new diet and do well during the transition period. Though may I also add that If you find his stomach is really not settling (& if he were mine I'd re-evaluate after 2-3 weeks) then it may be that his new food is not suiting him, as undoubtedly there are some foods that suit some dogs and some foods that simply don't.
Will you please let us know how you get on as this may help others too.