News that the U.S. Treasury Department would be replacing Alexander Hamilton on the $10 bill with a woman left many wondering why Andrew Jackson's face on the $20 bill wasn't being replaced instead. But Treasury Secretary Jack Lew says there's a straightforward explanation.

The $10 bill is currently scheduled for a redesign, and creating new currency is a painstaking process that takes years to plan and complete.

The $10 is the first in a planned new series of redesigned notes, Lew said on a press call Wednesday. It's a process that began before he took office, he said, and that he has been discussing with the Treasury since the beginning of his tenure in 2013. The new bill won't be unveiled until 2020.

It is a "happy coincidence" that the timing of the decision occurred soon after outside, private campaigns to push for replacing Jackson with a woman, he said.

Hamilton will remain on the $10 in some fashion, according to the Treasury, either on a separate $10 also added to circulation at the same time as the one featuring a woman, or as an image somewhere on the bill featuring the woman.

And it has to be the $10 that is changed.

"We have to do them sequentially, and it has to be based on security first," Lew said. The bills will include new anti-counterfeiting properties, as well as new tactile features to aid the blind.

Lew emphasized that changing currency is a lengthy process.

"If you think of it as an R&D project, you'd be closer to right than just thinking of it as just like running a printing press," Lew said, noting that there are complicated design and production steps in creating the new bill.

"It doesn't sound like you need to move now to have a bill in five years," he later added, "but my people are rushing me to decisions because they need the time."

Jackson was singled out for replacement in part because of his mixed legacy. Jackson's presence on the note has received scrutiny for his mistreatment of Native Americans and ownership of slaves.

Nevertheless, it's the $10 being redesigned to feature a woman, merely because it was next in line for an overhaul.