I had a similar situation come up while playing against my friends idoneth list:
he brought on his two units of 6 eels and rolled for charge with his soul scryer giving his bonuses. He rolled a 12 and I changed one of them to a 1 resulting in a 7 that can’t be modified - meaning he could only re roll one of the dice, nor apply the extra 3” to the charge (that’s how I imagine it working.)
However he disputed saying that the charge roll still gained the additional 3” because the ability came after the kairos ability with the wording “you MUST add 3” to the charge” with importance being on the ‘must’ part. I suggested that due to the most recent faq of the core rules suggesting abilities that clash, the latest supersedes the other - and that his ability occurs during the “beginning of the charge phase” while Kairos occurs “during the charge phase” suggesting kairos overrules. He feels different.
little help?