Wiki says the first version of these tools were released 45+ years ago so WinMerge is one of the latest on long line of similar software.
https://en.wikipedia.org/wiki/Diff#Unified_format
WinMerge is a good & free software one, and there are many other options too (Kdiff3 for example, is good for 3-way comparison and also free software).
BeyondCompare, SmartSynchronise are commercial but have extra features if one is doing this work in a large scale it may be worth it.
RE re-releasing: Personally, I wouldn't worry about it as this is just explaining how to make it work so that people can actually use the darn thing on their highres monitors (rather that re-releasing others work for monetary or other profit). Also who's going to sue you really, this is not Apple or Oracle to litigate every issue - this is just our cute but small scale subsim game!
If you are super worried about it and going to lose sleep, you could always release a patch file rather than the full thing; with this approach, rather than taking and modifying other's files you are just releasing the information that is a list of machine instructions such as:
go to line #7, delete first 2 characters, insert the following characters
go to line #847, delete 4 lines, add the following lines, and so on.
This is so that others (who have the original file like you AND who also have access to a software that can understand and apply a patch file in unified diff format) can easily fix their files based on your written instructions.
If I were you though, I'd just release it.
My suggestion, release away. If anyone complains about you showing people how to add highres screen support, you can take it down...