Late Change to Married on W2

Yes, it will be reflected on your return.  While you were still having your taxes taken out as single, they were simply taking out more taxes from your paycheck.  All of the federal taxes you paid in throughout the year are reflected in Box 2.

The way how it is reflected in your return is that since you are now filing as married (and I'm assuming it's a joint return), you get twice the deduction, twice the exemption, and twice the income allowable for each tax bracket.  In a nutshell, your tax due goes way down, especially if you have a single income, so you should see a higher refund this year. (And this would reflect the tax benefits you seek in your question).

However, if you do both work there are many couples who do prefer to have taxes taken out as if they are still single, and this is not only legal but probably a good idea The reason they do this is because when you are having taxes taken out from a job, that job does not know of any other job you or your spouse might have.  If each of you select Married, it is possible that not enough tax is taken out, and instead of getting a refund you could owe.  I doubt that's going to be the case this year, but it is something you might want to think about in the future.  Here's an FAQ that provides a bit more information for you:https://ttlc.intuit.com/replies/3301783

By the way, congratulations on your marriage!  Wish you many happy years.