I feed my Boxers on Arden grange and they are fine.
My pup has been on Arden Grange since she weaned from her mum.
You can change her diet, BUT please do it over a period of 5 days slowly decreasing the old food and increasing the volume of the new food until
on day 6 you have a bowl of the new food only. You can also add a teaspoon of natural live yoghurt to each meal. If you change the diet instantly you'll have a pup with the runs.
It is hard but keep her on a lead to take her outside to do her business (after feeding her)
and then let her have her mad few moments around an hour after she
has had her meal. Likewise don't feed her until she's been 'resting' for
an hour. So basically no exercise for an hour before or an hour after she's had her meal. This is done to cut down on the bloat/gastric torsion.
Also think about feeding her from a raised food stand. You can buy these from around the £10 mark.
Don't worry my Boxer who is now a year old also is a nightmare for eating things she shouldn't. She went through a stage of eating cats poo (yuk)
gets caught with countless things she shouldn't have. We've been down the vets a few times with the pickles she gets herself into (why you should always have pet insurance) We don't have any flowers within her reach (Hanging baskets only at the moment). She really lives upto her KC name of Ain't No Saint

You just have to keep telling her no-chew (take away what she shouldn't have) and give her something OK to play with, it will eventually sink in

Kind Regards
Kirstine