Replacing the helmet means even more merchandise sold, that's a typical marketing strategy in movies like this nowadays

The slight changes to Captain America's costume in each Marvel movie aren't here just to please the viewer. It means a same brand can release a Winter Soldier statue/figure, an Age of Ultron statue/figure and a Civil War statue/figure of the same character. More products, more money.
Kylo Ren's helmet was destroyed in the explosion, it's gone, and the chances that he is just going to build another identical one are slim to none.
A new Episode VIII helmet = a new Black Series Kylo Ren helmet (and these are insanely successful), a new Anovos helmet, countless new statues and figures, posters, t-shirts... And that's just taking into account the helmet, maybe they'll change the costume as well, even more incentive for people to buy the new products.