that's awesome!

What a great idea.  you  get new numbers matching the old, and you get a colored patch that makes it look like someone repainted.  Brilliant!

Nicely done!

I have some I need to re-stencil. I'm going to give it a try.

Weathering

Something to keep in mind if you give this method a try is that the printed numbers will likely not hold up if they get wet, as they could with some weathering methods.  I'm not sure how they'd do against a Dullcote overspray.  Just wanted to mention this so you had a chance to test it on paper and hopefully avoid having the ink run after it's already on the model.

I Disagree

Your method, while it works for you, is not one that I would personally use or advocate.  In the photos you posted, the numbers you replaced are completely different than the originally numbers in size and font style.  Using your method you should replace all the numbers in the reporting mark, which is typical for a re-stencil..  As Joe mentioned, the ink may not hold up well to weathering, depending on the weathering method used i.e. powders, oils, acrylics,airbrush,etc.  The thickness of the paper will create a 'ledge' that will catch powders and washes that could lead to unrealistic result.

An alternative would be to print your own decals.  Printing an inverse on white decal papers will resulting a white numbers.  One these cars it would be very easy since the cars are black and there is not need to match a color.  I would still remove the original numbers to prevent them from showing through.

JKTrains

Decal Paper

Another way I've used that's similar in concept, but more durable, is to create my own decals with dry transfer lettering and coloured decal paper.

From over 30 years of purchasing, I have a large personal supply of C-D-S dry transfers from various railroads (mostly the CPR) and general alphabet sets. A few years ago, I figured out that I could just as easily rub the transfer lettering directly onto decal paper, and make my own decals for easy paint patching effects.

Microscale makes decal paper of many different colours (and clear too of course) including black. I'd start with that, and carefully rub appropriate dry transfer numbers onto the black sheet. Then cut apart a square of the sheet containing the new numbers, and spray a clear coat of paint to seal the transfers (you can use a can of Dullcoat, or spray any clear finish with an airbrush) ...and voila! ...custom patch decals!

I recommend you replace the entire road number, just to get the font consistent. However with these decal paint patches, plus weathering, people would be hard pressed to tell that they had ever been patched. And even if there was a difference in finish, it's not like real freight cars don't get renumbered sometime in their service lives.

The key to Dull Coat is several thin coats!

Applying several thin coats with the first coat or two should be sprayed at a greater distance so it goes on "dry". repeat this a couple of times and then your final coat can be normal.  As I said, this method is just another way to do something that for me brings satisfactory results.  If you look real close, you can tell it's paper, but my point in doing this is for operations and having 4 cars with the same number needed a quick fix.  Well, I say quick fix  but as some of these freight cars are now over a year old, they have held up pretty well.   I weather most of my rolling stock with an airbrush and some with powders and add details with Pollyscale paints and haven't had any issues.  Besides, it's areal easy fix using this re-stenciling method!