Changing the Aggregation Selection in a Check Can Cause the Check to Fail

1. Have a measurement where the field value is an integer
2. Create a Check, and leave the default aggregation to mean()
3. Let the check run once
4. Reopen the check, and change the aggregation to max()

Result:
The check fails because Max() now returns an integer, and the Check task tries to write an integer to a field that was first written with a float. This information is available by searching the _task bucket.

Expected:
The Check does not fail. At a minimum, provide an error message so the user has a hope of figuring out what the problem is.

From the task log:
```
"message":"could not execute task run; Err: partial write: field type conflict: input field \"n\" on measurement \"statuses\" is type float, already exists as type integer dropped=1: partial write: field type conflict: input field \"n\" on measurement \"statuses\" is type float, already exists as type integer dropped=1"}]
```

Reproduce the issue by changing a Check aggregation from mean() to max() after one run, then inspect the _task bucket and task log for the field type conflict. Trace the Check task behavior around writing the changed aggregation result; done means the Check no longer fails, or reports a clear error explaining the type conflict.

Written by the indexing model from the issue text.