Since Varget seems to be out of stock here in Europe, I might need to re-do my established load.
I am using Hornady 178 BTHP with 43.8 gr of Varget, this gives me approx. 2550 fps from a 20" bbl (for 308 btw)

I am thinking about VV N140, cause I have plenty around.
In any burn rate chart I've seen, Varget and N140 are pretty much next to each other.

For the sake of cost, time and material I would like to hear your opinion on how to approach this, without starting the whole load dev from the scratch.
Maybe starting 0.5gr or 1 - 2% lower. Of course all other components would stay the same.

If I try to reach 2550 fps with N140 (within all given safety manners), can I expect similar results to what I was getting with Varget?
Or is pursuing the same MV the wrong approach?

Input highly appreciated.

I would start from scratch.  Behavior as volume of powder increases is not necessarily the same.

Try:

RE-15 or 203-B

VihtaVuori N150

IMR-4895

IMR-4064

All of these powders should work well in your rifle. They all are perfect for semi-auto's.

The first three listed should be easy for you to find.

VihtaVuori N140 is slightly faster than Varget. I would start at least 1.0 to 1.5 grains lower to start.

It should work well for your application. I like it for 5.56mm.

Be careful not to mix military surplus brass into your reloading components. It must be loaded with 2.0 full grains lower powder charges because of reduced internal capacity.

R-15 and never look back

Because your exact bullet, powder, primer, case, and rifle, don't match their data, you should back down before your attempt the swap.

Your load doesn't show in the V N-140 data, but two other loads with a 175 and one with a 180 do show and your load is at the top of those ranges.

It isn't advisable to start out at the top with a swap.

Your development loop should still be very short, so I would relax and make the first test a walk up with the chronograph and see how it goes.