Some owners are behind on assessment payment plan

Special assessment was administered giving owners an option of pay all at once or pay x dollars over a 2 year period. A handful of owners have paid zero now that we are 4 months into the assessment time. Can the Board vote to change the assessment terms so that those who have fallen severely behind on the 2 year payment plan are made to pay in full. We thought we were helping a few who are in hardship - but don't want to live in a collection nightmare. Interestingly its not the hardship people falling behind. Some who are clearly able to pay in full have chosen to take the plan and then paid nothing for 4 months. Can we change the terms now with a Board vote?