Very slightly, but yes it will.
There is a thing the OP needs to watch out for, that the threads aren’t unusable due to carbon build up in them.
I would be screwing the plugs in very carefully and/or using a plug chaser to clean them out.
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That is a very good point…
You’re right, I was putting it off a bit 
I was very careful putting them in, just naturally a bit paranoid of ruining threads. There was some amber stuff that crumbled a bit taking the old ones out that I could swear had bubbled out of the older plugs, but I could be mistaken.
Well anyway, I started it up this afternoon, no problem! And went for a good one hour spin. Definitely has fixed the original issue! Can easily cruise along in any rpm I like and it just holds it dead steady. Wasn’t possible around 2000-2500 rpm before.
I don’t know if it pulls any stronger or anything as it’s new to me, but it seems to feel so much nicer to drive enthusiastically at higher revs, I think it’s because the overrun/deceleration is just so smooth now.
So a big thank you to the encouragement and reassurance of everyone in this thread getting me back on the road 
Feeling a lot happier about my new motor now.
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Now it’s running better just remember to keep a close and regular eye on engine oil as they can use a bit and don’t tolerate low oil level well.
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To me it would be obvious that the heads would be “slightly” different between the 1.8 and the 2.0 somehow, (Because they do have different plugs, pistons, bore size etc).
So I contacted Mazda to ask the difference and was basically told “That they are different”!
***We are unable to provide the exact differences between the cylinders as this information is Mazda corporation intellectual property which cannot be shared outside of Mazda. ***
What I can advise is the cylinder heads between the 2 models do have different characteristics due to the cylinder bore on the 1.8 engine being smaller. This change in bore size and piston leads to different spark plugs being needed between the 2 engines.
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