1- I appreciate last game update making research cheaper for mbts but making building them cheaper is another story.
mbts usually out perform afvs but they are less mobile (speed and and late airlift) and was more expensive.
Now they are somewhat comparable in cost and mbts is a much better choice so ..
I think yes to the research reduction and no to the build cost reduction

2- why does tank research change cost (without changing tier).
Tanks research line is hmf not the best now this is another point to discourage tank upgrades
Usually I change tier when I have good amount of units from the previous tier.. now upgrading tanks doesn't seem to be the best idea.

I mean mbts tier 2 are lvl 4
I'd keep building mbts/afvs at lvl 1 ,2,3 and upgrade them.
Now I can't I ahve to build all the tanks at tier1 which is very discouraging.

I think last mbt cost reduction gives a huge nerf to afvs being in many cases inferior .
In the past I thought of mbts to be defensive choice (on open grounds) and afvs to be the aggressive one now I can go mbts all the way regardless of the 25% city modifier

this design might give rise to researching more types of units instead of upgrading specific some. I still don't like it but they have their measures for balance and we will see.