I knew you could do it, I just had no idea how. So I asked Reddit and they came up with this
When you reach the Novus stage, you meld materia on the scroll. Once you have all 75 materia melded, you turn in the scroll and the Animus to Jalzahn and get the Novus in return.
After that point, you can buy another scroll, talk to Jalzahn, and select the modification step for the particular Relic stage that you currently possess. He’ll copy over the current stats of the Relic (the ones you melded previously). You can then remove stats one at a time at the Bonfire and then redo the meld. You will need more Alexanderite for each meld. Once you’re finished remelding, talk to Jalzahn again to copy the new stats back to the weapon.