A longtime lightweight, Johnson, 31, hasn't weighed the featherweight limit of 146 pounds since high school, but says the process of dropping down has been a positive one.
Still, he admits there is some anxiety thinking about cutting those final pounds.
"I have nightmares about it every night," Johnson told ESPN. "You have no idea.
"My first fight at 155 [pounds], I got stuck trying to lose the last three pounds. I couldn't get them off. That's what I keep having flashbacks of when thinking about getting down to 145. What if I get stuck?"
Despite the nerves, Johnson [17-12] doesn't anticipate any problems. He says he only weighed around 163 pounds during camp as a lightweight, and always believed a drop to 145 pounds was an option.
He's working with renowned nutritionist George Lockhart and says he's "all-in" on staying at 145 pounds.
Fighting out of South Florida, Johnson is 1-4 in his past five fights, but many of those losses were close and came against the very top of the 155-pound division.
Johnson doesn't believe he'll need a lot of time to shake up his new division.
"I've spent my time, paid my dues in the lightweight division," Johnson said. "I'm not looking to work my way up [slowly]. I want to come in here and beat [Elkins] who is on a nice win streak, make a statement, and then get in there on a No. 1 contender fight."
Johnson, who is originally from St. Louis, initially called out bigger names than Elkins (23-5), but says he's happy with the matchup as Elkins is currently riding a five-fight win streak.
As long as the cut continues to go well and Johnson gets his hand raised on Sunday, he's already looking at the very top of the division -- champion Max Holloway and title challenger Frankie Edgar. Those two are scheduled to face each other at UFC 222 on March 3.
"Frankie is on a different level that I don't think Max has seen yet," Johnson said. "I think Max has gotten away with beating a Jose Aldo that really wasn't in the game. I think he's gotten off on these last two fights a little easy."