So help me understand your goals. You have a Legacy IBB backup plan with a chain of incremental backups and you are looking to convert that to the IBB plan with the New Backup Format that allows Synthetic full backup?

Yes I have an old Cloudberry job that was setup as only Incremental pushing them to S3.
In S3 I have over a year of incrementals sitting there. I would like to change this backup plan to include a weekly Synthetic full. I was concerned if it would pickup that change and convert all of the incrementals that we currently have to a synthetic full upon making that change in the plan.

I have done the update to the New MSP360 Ultimate Edition (formerly Cloudberry)

job that was setup as only Incremental pushing them to S3.
In S3 I have over a year of incrementals sitting there. — Robert Wilkerson

Do you mean it's a file-level backup and you don't have a full backup with this plan?

Yes we are backing up several different folders. (Not Entire computer).
There is no fulls setup on this. In S3 there is 1 full backup and a years worth of incrementals

Question: What is your retention period requirement for versions and deleted files?
Since you have not run a full since day 1, you have saved every single version of every modified file for an entire year. We do Monthly incrementals (aka fulls) and daily block-level incrementals, with a retention period of 90 days. Keep in mind that with the legacy format, a "Full" backup is not a complete set of all of your data (like it would be with the new format using synthetic fulls).
A legacy format full backup simply does a backup of each individual file that is new or has been modified since the last full backup. It never has to re-backup files that don't change such as pictures or pdf's.
My recommendation would be to go into the schedule and set the retention to whatever number of days you want to keep old versons of modified files, and set "keep deleted files" to that same number.
Set up an advanced schedule to do a monthly "incremental" full and daily "block-level" backups.
Would be happy to discuss at greater length if you have more questions.