Coffee lake only has a 2 year lifespan max so doing it in 2 hits would be pointless,by the time you come to changing the cpu it would likely be discontinued.
The i3 8100 does have significantly better single core performance but dropping 4 threads makes no sense at all - for gaming it will essentially be worse in multi threaded titles.
The i5 8400 is an upgrade ,the i3 8100 half is & half isn't - its just not a sensible choice.
If the main priority is getting onto a current gen platform without losing performance then a ryzen 1400 at stock has similar performance to a 3770 & when overclocked can be significantly better.
Thats the cheapest cpu Id recommend to get you on a new platform without compromising the performance you're...