They didn't - i hear you on most engines, but they specifically mentioned with the 2.5 they actually scaled the engine up vs just boring/stroking. Different sized block and bolt ons, etc. i'll try to find the source - i know it was Dave Coleman discussing it in some interviewUsually, the engines from the same family are just bored/stroked. The head is interchangeable. Again I'm referring to Mitsu 4Bx engines. 4B10, 4B11 and 4B12 varying from 1.8 to 2.4 use the same block, which is bored and stroked, so the heads, intake & exhaust manifolds, camshafts, etc. are interchangeable.
I really hope that Mazda used the same approach.
here it is...cool interview overall actually - he says only the rocker arms and cam gears are the same between the two.
edit: and finally, i see my fellow forumites already helped with this thanks!