Started this in armourers section but need more info please.

Picked up a 225A, great shape but night sights are dim. Front is clearly a 6. Rear is tough to read and looks like a 6 also.
When shooting, I have the front sight covering the poa. That's where it groups.

If I change the rear to a 8, will it allow the poa to be just over my front sight?

Changing a Classic SIG pistol rear sight from 6 to 8 will raise the point of impact about 4 inches at 25 yards. It will be proportionally less at shorter distances; e.g., ~2.4 inches at 15 yards, ~1.12" at 7 yards, and so on.

But a couple of additional thoughts.
I would be surprised if the rear sight were a 6. It’s not impossible, but long ago SIG evidently got away from using different sight number combinations in an attempt to carefully regulate the point of impact at a specific range. Before going to the trouble and expense of changing the rear sight just in hopes of raising the POI, I would try to confirm what your current sight is. If you can’t make it out using good magnification, perhaps a call to SIG customer service would be worth it.

And also FWIW, SIG doesn’t even list a #6 rear sight for sale on their site any more. At one time there was a whole range of front and rear sights with different numbers available, but that hasn’t been true for many years.

At one time at least, it was the official claim by SIG customer service that the proper sight picture for their Classic pistols was to position the dot/lamp of the front sight over the desired point of impact. Some people have called that a “combat” sight picture. Your sights may therefore be regulated for that concept.

Plus something else: According to Wiki, the P225-A1 production started in 2015, or no more than nine years ago. With a tritium half life of about 12.5 years and with night sights being about half as bright at that point as when new, I always question what is meant by their being “dim.” After nine or less years the sights will be dimmer than new, but what’s our expectation? I just checked a set of sights that were new on a pistol in 2011, and after letting my eyes become dark adapted for literally just a couple of minutes, the night sights were bright enough to be usable. Have you compared your sights to a newer set?

Seems like having a fixed rear sight would be easier for the layman to adjust the POI by just changing the front sight height. I don’t think Sig offers a rear sight of different sight, I could be wrong