As long as you communicate with the client and make sure the two of you are on the same page, then yes, this is safe.
Here is something to keep in mind:
Upwork employees are NOT GOING TO CHECK the files you submit, and then read the milestone description, and then verify that you did the work that was requested.
EVERYTHING is between you and the client.
If your milestone asked you to use a red border in the milestone, and then messaged you to use a blue border, and you submit the file with a blue border...
...Then the only thing that matters is that the client is satisfied. And then she releases payment.
If your client is honest and releases payment as expected, you don't need to worry about what is written in the milestone description.