670 mbs vs 620 mbs

They're blades. They're practically the same whether they're from 1980 or 2024. The only thing they do is move the CG around but won't have any noticeable effect to you unless you strike it like Tiger Woods. They also have ground the soles so the turf interaction is different. Likely something the average golfer wouldn't be able to notice unless... they strike the ball as consistently as Tiger Woods.

The Pros will notice differences, but they are so consistent they can tell when the club interacts with the turf differently. There isn't ground breaking change in performance between a blade from 2024 and one from 1970.

You can only tell a significant difference in feel if its a different manufacturer that uses a different metal.

Yep - blades are blades.
Based on specs, 620s are 1* stronger lofted (not much overall) and maybe they've changed bounce, offset and CG slightly.
I would definitely have the lofts and lies checked on those 670s, so you can be confident they are as you would expect.
Nothing worse than struggling with them on the range, thinking "I can't hit blades", only to later find out specs are way off.

Good luck - enjoy the journey!

Differences are subtiles, often, the feel is subjective,
1) more soft or crisp,
2) more or less solid at impact
3) center of gravity lower or higher for a different trajectory
4) different soles giving a different feel through the turf.

I did not played the 620 mb but I only read excellent comments on them. I do not know how Titleist will do to replace them.
In the past I played miz mp 33, mp37, Titleist 680mb, Titleit 710mb, Titleist 714mb, Miura baby blades. And all of them has a different feel.
M’y conclusion is, differences are not nights and days but you could feel the differences.