Re: Update the outfit for next solo film
I dont think an explanation with the suit is even needed. I think the audience would accept a different costume in the same way that they assume Iron Man will have new armor for every movie - just as IM upgrades his armor, Cap is constantly being provided with upgraded costumes/outfits which are improvements on each of their predecessors but may change a bit stylistically.
Don't be ashamed of the source material when making your adaption. EMBRACE it.