Look closely at the early and late dates of your affected tasks.
According to your description (and ignoring other impacts), at least one of your successor tasks already has a logic-driven early start date that is 9 days later than its SNET date.
SNET constraints on the successor tasks only have the potential to affect their early dates - with no effect on your project start milestone.
By changing the SNET constraints to MSO, you are completely over-writing both the early dates and the late dates on those tasks. The backward pass carries this effect back to the late dates of your project start milestone.
MSO/MFO constraints in MSP are similar to Mandatory constraints in P6. I avoid them in general. If Total Slack or the "Critical" flag is to be relied on, then you must never assign them to any logically-connected task. I wrote more about this last year: http://wp.me/p6CCB4-7w
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18 years 11 monthsMoutaz,Look closely at the
Moutaz,
Look closely at the early and late dates of your affected tasks.
MSO/MFO constraints in MSP are similar to Mandatory constraints in P6. I avoid them in general. If Total Slack or the "Critical" flag is to be relied on, then you must never assign them to any logically-connected task. I wrote more about this last year: http://wp.me/p6CCB4-7w