I set up a basic project with three tasks where each task is configured to be one of the basic task types:
- Fixed Duration
- Fixed Work
- Fixed Units
I then copied the tasks down and grouped then into two groups: Base Plan + Modified Plan.
Base Setup
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As you can see, there is a task of each specific type and each task has a duration of 2 days and a work amount of 16 hours. I then assigned a resource RA to each task.
Modifying Fixed Duration Task
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After modifying the Fixed Duration task to 24 hours only the Work column was highlighted. No error messages, just a little green triangle telling us that Project has altered something.
Modifying Fixed Work Task

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After modifying the Fixed Work task Project highlighted the duration and end date and provided the little green arrow to hint that I could alter the default behaviour of how Project handled the change.
Modifying Fixed Units Task

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After modifying the Fixed Units task by adding 6 hours to the work Project highlighted the duration and work columns. And again Project lets me know with the little green arrow that I could alter the default modifications to something else, by clicking on the cell and then selecting the exclamation mark symbol to the left of the cell.
Automatic Planning Mode
All these changes were conducted in automatic planning mode, so Project will adjust according to its defaults.
Resource Impact
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Adjusting the Fixed Work and Fixed Units tasks resulted in a longer duration of work (3 days), which is the default handling when adjusting the work amount. However, adjusting the Fixed Duration task resulted in a (default) adjustment of the units (resource allocation). The employee RA now has to work 150% on two days of the week to accomplish the **Fixed Duration* task.
Base Formula
The default adjustments are all based on the following formula:
1 day duration = 8 hours per day / 100% Units per day
If we apply this formula to the Fixed Duration task this will result in:
2 d duration = 8 h work per day / 100% units per day => 16 h work / 100% units
We add 8 hours to the work which results in 24 hours in two days, because duration can not be changed:
2 d duration = 8 h work per day / 100% units per day => 24 h work / 150% units
Because the value on the left side of the equation cannot be altered Project has to modify the only value on the right side of the equation that can be altered and that is the value for units (which is the equivalent of resources).
16h / 100% = 0.16h / 1% unit
Which is the same as:
24h / 150% = 0.16h/1% unit
To answer your question
Your error message is not the default behaviour when modifying a Fixed Duration task in MS Project. Project will increase the resource percentage if the task is configured as a Fixed Duration task.
The task type might have been set as Fixed Work or Fixed Units type.