...don't do it on your actual 2018 tax forms..leave that alone..make sure it is back the way you started back the way it was hen you filed...and saved that way
.....and then do a Save-As to some test file name. and only mess with that.
when you increase your box 12 amount, you must also reduce your box 1 value. the box 1 value has the major effect.....the box 12 value for that particular entry is primarily an explanation for why box 1 is what it is. Other box 12 values can also affect box 1. So if you increase your box 12 code D by $1,000, you then reduce box 1 by $1,000
____________*Answers are correct to the best of my knowledge when posted, but should not be considered to be legal or official tax advice.*