|No such thing as a small change|
You've already answered #1 and #2, just bear with me.
(1) Were you assigned to make the tool BY the future users? (Or was it by some lead figure or boss?)
(2) Does your tool get exactly the same results as what they got, only with fewer steps?
(3) If not, then why did you make a tool for them?
You can't change other people's behavior. They'll use the tool if they feel like it. Most people don't want to change their existing methods for some new method they haven't used, even if they don't like their current approach or results.
Update: If all of your managers sign off, and you decide that it's for the best, then "break" the other users' existing methods: change the system so their old methods won't work anymore. They'll start coming to you with the "now, how does this work again?" questions.